Hm. I don't remember everyone being super mean in the BBC's Robin Hood (2006), or in A Knight's Tale (2001), or in Hunchback of Notre Dame (1996) or Notre-Dame de Paris (1831), or in Lord of the Rings (1954)...arguably, your examples indicate that people are super-mean today, not that they were back then.

That said, there's a lot to be said historiographically about the development of the Middle Ages as The Crap Ages, a whole host of ideas that play into a general, pre-GoT modern idea of the bloody medieval period. However, in terms of "scholarship," the theory is most closely tied with Steven Pinker. In Better Angels of Our Nature and Enlightenment Now, Pinker argues that the world has been on a trajectory of progress and improvement since the Enlightenment--less violent, higher standards of living, better opportunities for wealthy Westerners people to reach new intellectual and technological heights.

Pinker is a psychologist.

Pinker is not a historian.

Many others have eviscerated his arguments piece by piece (and I mean, even in popular publications), so I'll pick out a few examples that work on different levels.

If you are going to start yelling "But Pinker has DATA," please skip to the edited-in addition at the bottom. Thanks!

First, historical evidence. Pinker loves to cite falling murder rates as a sign of "moral progress." I'm sorry, but what? Murder rates? My dude, we don't even know if 1300 France had 10 million or 20 million people living there. And you want to talk about murder rates?!

Second, perpetrators of violence. One of the reasons the Middle Ages appear so violent--including in historiography! because they were, in fact, quite violent--is that the violence is chaotic. I've talked before on AskHistorians about how "bandits" were less often outlaws, and more often completely legitimate nobles and their retainers. Building castles in advance positions to use as bases for raiding parties, burning villages and crops of their rivals, kidnapping travelers for ransom. When 14th century priests are writing treatises on the seven deadly sins, their example sins for "wrath" are war, murder, and arson.

I am sorry to semi-Godwin this, but it's the perfect example (and it's not "because Nazis, QED," I promise). Late medieval Europe had horrific pogroms--you might say late medieval Europe invented anti-Semitic pogroms. But you know what it did not have? A Europe-wide, genocidal Holocaust. Not because there wasn't that level of hatred. I've read the sources; there absolutely was. It's sickening. But what the modern world has that the Middle Ages didn't? Institutional apparatus to carry out that violence on a terrible scale.

Medieval Europe had a bunch of lords running around fighting for territory and it's just too bad whatever peasants got in the way and whatever women were there to be raped. The modern world has streamlined and centralized that violence into the hands of the state (obvious exceptions are obvious). Medieval violence was personal; modern violence allows us distance: a "home front", or even more muted, yellow ribbon magnets on the backs of our cars. That's not better or worse. It's translated.

Third, on the theoretical level, much of the modern debate turns on a scholar named Norman Elias and his Über den Prozess der Zivilization--the "civilizing process" thesis. Elias argues for the decreasing chaos and external violence of society, which Pinker cites with praise. Pinker does not cite the other half of Elias' theory, which is that "civilizing" is not some bountiful "happiness-increasing." It again turns back to the order/chaos perception, except in this case, you might say the focus is on external violence become internal. We police ourselves and each other through means less outwardly bloody, but inwardly much more strict.

Fourth, and less Pinker, more medievalist: our modern perceptions of the Middle Ages just kind of...leave out a lot of the nice parts. I want to offer a couple of examples not to say "The Middle Ages were kind and fluffy," because they weren't, but to give you a flavor of what's there that we don't realize. We don't see bishops gathering their city's Jewish residents into the walls of their castle when they know a murderous horde of Crusaders is about to storm through. We don't see hundreds of women and men writing to Hildegard of Bingen for advice, like some kind of medieval Oprah or Dear Prudence, and we don't see her writing back to them that yes, she has seen in the Living Light that their deceased children are in paradise, they can rest easy now. We don't see Italian confraternities whose members spend nights with convicted criminals before their executions, praying with them and comforting them, who accompany them right onto the scaffold so they're not alone in their last terrifying moments. (Or the executioners who skip all that and just get people well and drunk).

So throughout this post, I've used "Western" examples to criticize this theory, operating under Pinker's paradigm in which the Western Enlightenment is the driver of everything good in the world. I don't want to dig too much further in this answer because (a) modern politics and (b) in fairness to OP, who asked about medieval Europe in the first place! However, it's worth considering that the ease with which Pinker assumes that the Western, specifically, Enlightenment is the driver of progress for everyone in the world is itself something that might be a sign of a certain type of civilizing process--and what a more global perspective might say about these various angels of our nature.

ETA: For the "but DATA" people:

Pinker’s thesis is based on data, yes. However, as clarified many times here and elsewhere, history and science are not just data. They are interpretations of data. This is where Pinker fails. The argument contra him is not for less violence in the Middle Ages. I don’t know how that can possibly be the takeaway of my post. The argument is for the transfer, displacement, translation of violence today. This comes in many forms, and I'd suggest you take a look at murder rates in different countries today.

And as a historian, I understand the importance of context. London in 1300 is not the same as London in 2000 in terms of their overall place in the global politico-economic world. This is a crucial factor to consider in terms of crime rates, including murder rates. Pinker cites for comparison murder rates in “stateless societies,” but fails to consider that anthropologists’ studies also take place in a post-Euro-American imperialist world.

The point about a global context is also vital to considering the aspect that Pinker glosses over about those medieval murder rates as calculated by Given. They, uh, are actually significantly worse for the 14th and 15th centuries than the 13th (where Given starts). Like many other “crisis of the late Middle Ages” phenomena, this was once atteibuted to the Black Death but has more recently been shown to be a much longer-term, slower development rooted in socio-economic changes. Urbanization and increasing ties into Hemisphere trade; population decrease, and so forth.

There’s a whole lot of story here that Pinker is missing completely when he says “numbers A, numbers A prime, numbers A minus numbers A prime.” He’s presenting an interpretation of data that is methodologically flawed.

The argument is not for less violence in the Middle Ages, at all. The argument is that Western white people like me, whose lives are infinitely better than they would have been in 1300, have displaced violence from ourselves onto Those People that we don’t have to see or think about. And that is the problem.

On the subject of bishops gathering the city's Jewish residents behind the castle walls, the way that medievals recorded history is one component of how this impression of a violent middle ages came to take hold. In a popular sense, we tend to conflate history with fact-finding, but that's a modern idea. There are very different goals behind something like a chronicle, which might be regarded by an amateur as a direct historical record (and remember that the foundation of medieval history is in the work of 18th and 19th century amateurs). A chronicle provides a record of significant events meant to be consumed by future generations. One intention is that the future generations will know what happened to their community so that they can orient themselves in the world. Consequently, chronicles tend to have a disproportionate number of stories with morally satisfying conclusions (generally stories of wrongdoing being punished or order falling to chaos and being restored). Did the chroniclers select satisfying narratives over unsatisfying ones? Did they tweak historical events to increase the human interest angle? Probably both, but either way, we're looking at a record distorted to maximize wrongdoing and chaos (and the resolution thereof).

The other major consequence is that these record only "significant" events. What are significant events? Battles, disasters, famines, changes in power structures, sensational crimes, the enactment of a major treaty or charter. What are insignificant events? Just living your life, man. Peace and prosperity. The laughter of children. Early medievalists--who did not work under the strictures of contemporary historians--tended to take these accounts not necessarily at face value, but with different kinds of skepticism than we would today. They lacked our wealth of archaeological data and modern tools like collective biography, which historians today use to fill in details of the "just living life" history. As a result, their narratives of the middle ages leaned on these "significant" events to the exclusion of everyday details.

The consequence of those omissions isn't merely that people imagine that medievals were "super mean," but that the overarching narratives of these highly elliptical chronicles are still with us. The historian Magda Teter observes that medieval Jewish people primarily appear in chronicles when they are accused of blood libel or when they are victims of mob or state violence, and consequently uncritical readings of these sources continue to inform a default historical position that Jews were outsiders to European society:

In scholarly studies, including textbooks, the Middle Ages have been represented as a Christian story. Jews, if they are present at all, appear as a hated and persecuted minority, engaged in usury and other unrespectable activities. For example, in a book Economic and Social History of Later Medieval Europe published as recently as 2009, Stephen Epstein presented Jews as a separate group, maintaining their own language and culture. The book emphasized the Jews’ “exclusion,” described them as “scapegoats,” and noted “massacres” of them. Monarchs, for example, “squeezed the Jews to skim off the profits, when the Jews could yield no more they were expelled.” This was the case in England, and France where Louis IX’s bureaucracy “went to work and increased regular tax income. Jews were particularly hard-pressed—money lenders were expropriated and expelled.” Epstein, perhaps unwittingly, used nearly the same verbs as the medieval and early modern chroniclers, without integrating Jews into a larger story as key and integral actors in the European past.

("Blood Libel, a Lie and its Legacies," in Albin et al eds., Whose Middle Ages? (Fordham University Press 2020). Disclosure: I am publishing this book and this post is heavily indebted to this specific essay)

One of the problems typical for the portrayal of the fictional events set in the historical (or realistic settings) is that the events are usually heavily skewed for the entertainment and this is on of the reasons that it is called 'fiction' in the first place. And even if something is called a 'fictionalised story' or 're-telling of the actual events' it usually means that the portrayal will more often than not pay no more than a lip service to the facts (I'm not using the term 'history' as it would apply to contemporary settings as well). For example, let's take the case of 'Breaking Bad' series that takes place in Albuquerque, NM over the course of slightly more than 2 years starting in 2008. According to my rough calculation, the series portray slightly over 60 murders committed in the city, excluding all homicides that were committed in other places, were not murders (e.g. people shot in self-defence) or could have not been discovered or ruled as murder (poisoning, staged suicide, well-hidden bodies). Meanwhile, the whole yearly homicide count in the Albuquerque in late 2000's was whole 70 death per year what means that few characters shown to kill all these people would have been responsible for every second homicide in the city. Possible? By all means, yes. Probable? Not really. But this is exactly how it was made and you can only imagine that if such a portrayal was not considered a grave exaggeration despite murder rates in US cities being common knowledge, at least to Americans, you may only wonder what wild exaggeration would be give a free pass in the portrayal of historical events and milieu, especially one not well documented (as u/sunagainstgold wrote below, making something as simple as calculation of murder rate in 14th century France is beyond the capabilities of experts who aren't sure how many people lived there in the first place).

Another good example is the way many discussions related to the historical veracity, you may often find the laymen opinions based on the correct portrayal of some artifacts from the corresponding period what, from historian's point of view is maybe not insignificant (proper dating of artifacts is important in studies of some details) but of secondary importance at best. For example, I often find the opinion that the 'Kingdom of Heaven' is historically accurate because the weapons and armours used correspond to ones actually used in Holy Land in late 12th century, even though everything else, from the social interactions, to expressed motivations, to politics is usually a projection of modern attitudes and sensibilities to convey a story that is believable and comprehensible to a modern audience. If you're lucky, you may not even find a more or less veiled modern political commentary. It is not bad per se, on the contrary, all of this makes a good story. But it will not be a HIstory. History is all about documentation of past events. And documentaries are generally considered boring and non-captivating, at least not to an average reader or viewer.

How this all relate to the original question? Well, in short, fictional portrayals are hardly about real people. Real people tend to be mundane and boring, so they hardly the material to spin an epos about. Please take note that characters in 'Game of Thrones' or 'Robin Hood' are predominantly nobility, often aristocracy or knights with a commoner here or there to play a foil to the majority of the cast. In reality, depending on the area and time, 75% to 90% people living in the Middle Ages were peasants who spent most of their lives working the fields, carousing on holidays and praying to saints to spare them from disease, famine and war. Furthermore, most of them were spending their entire life in one village and its close vicinity, so the journey to the nearby city to sell the products was an uncommon event and a rare occasion to visit other settlement. In other words, your average member of medieval society was spending their entire life among the same 200-300 people (and their parents who died in the meantime and children who were born later). Such milieu is hardly conducive to rampant crime (most people knew that relatives of the injuried party will know the perpetrator and will seek justice) and given that medieval villages were pretty much self-sufficient, people had to know and support each other to a large extent (this is why expulsion, an apparently lenient treatment could have been fatal as it essentially reduced the sentenced party to begging and vagrancy). Anonymous contacts, commonplace now, were basically impossible due to lack of means of communications. Most peasants usually knew their officials and landowners personally (usually because the latter was often had a judicial privileges and thus was presiding occasional court proceedings involving his peasants), so this extended also to the inter-class relationship. Even in the cities, people were usually living in the same place for decades (several generations often using the same house) doing one job from their admission to the craft in their youth until their untimely death or elderly age when they were no longer able to work. Same area, same work, same people around. This actually strengthened the social bonds rather than dissolved them.

Of course, this does not mean that crime took no place, far from it. Middle Ages saw its share of thefts, arsons and murders ('Records of Mediaeval Oxford, Inquests of Coroners, Walls of Oxford etc.' edited by H.E. Salter and published in 1912 gives a nice insight into the latter), much like any modern city. It should be noted however, that the latter usually stemmed from low level of medicine development and care available and thus a significant number of what would be a battery or assault today ended with death due to fatal brain oedema or sepsis, more often than not two or three weeks after the initial assault, as posited by Manuel Eisner in 'Violence in Evolutionary and Historical Perspective' (the study is another example of problems mentioned above as the actual population of 1330's London was estimated with 50% error margin what amounts to a trifle number 40.000 people). Another related subject was the fact that lack of communication made secluded places far more dangerous than nowadays with possibility of being accosted by bandits being incomparably higher than in modern developed countries (in Europe it is extremely rare of and usually limited to organised criminals hijacking trucks carrying valuable goods). But again, we need to put things in perspective, because in most places crime is rare. Even if the crime rate was 10 or 15 times higher than today, it doesn't mean that 'everyone was evil', as this is the ratio between, say, intentional homicide rate in United States (5.35/100.000) and China (0.62) and Japan (0.28). Does it means that living in USA is in some way horribly dangerous? Not in the least. Or, using the Eisner's example above, the proposed murder ratio in London in 1300-1340 being 15-20 times higher than in contemporary London (the paper was published in 2009) corresponds to the murder rate of 20-24 (this metric for London was 1.2 in 2017) what corresponds to the rate of Philadelphia, Cincinnati or Chicago as of 2017. Neither of these is known as particularly dangerous (well, the latter has some associations with organised crime, but these are mostly historical). Baltimore, New Orleans and Baton Rouge show twice as high murder rate than the metric for 14th-century London proposed by Eisner. But even these cities are hardly places where 'every regular citizen is [...] a giant asshole, everyone is ready to beat you up and steal your stuff and murder is [...] almost common place'. This is because dramatic works of fiction focus on dramatic elements and filters everything else (read: normal, boring people that form ninety-something percent of society) out of the picture.

So, yes. Portrayal of Middle Ages as extremely violent times is just a way to sell a compelling narrative, nothing more.