It was no doubt a grim time for many, but that's an entirely subjective opinion. It is important to remind ourselves that the past was a violent and unfair place, so much so that I don't think the 'Dark Ages' were any more or less 'barbarous' than any other period. The majority of the population, whether in the Classical period or during the Middle Ages, worked in the fields and had nothing to do with philosophy, art, or high politics, at least in any way that we can study. Our sources were overwhelmingly written by the elite for elite consumption; as a result, when we think of Rome we think of aqueducts, of great military victories over 'barbarians', and of a wealthy literate class prospering under able emperors, whilst the 'nastier' aspects of their rule don't come to mind. Their empire was built on military conquest and political subjugation, each conflict followed by looting and what we would consider to be war crimes, and their economy helped significantly by new influxes of slaves. In that regard, they were no different from their 'barbarian' successors during the so-called 'Dark Ages' - I really struggle to think of anything particularly bad that happened in this period that did not have any precedents before or parallels afterwards. People, I think, are just people, and it is difficult, if not impossible, to qualify any group as better or worse than another.
Peter Brown, a pretty big name in the study of late antiquity, summed this up quite well, though admittedly this dealt with the late Roman empire. Even so, the same is pretty much applicable to any period of history:
Most persons lived miserable lives, at a standard of living that never reached beyond that enjoyed by the populations of other preindustrial empires, such as Moghul India. Like Moghul India, the Roman empire was a colorful place. But the color wore off very quickly as one descended the social scale. We should always bear this in mind. The noise generated by a vivid Christian discourse on wealth and poverty (written by members of the curial class and largely addressed to the inhabitants of the cities) affected only a small proportion of the overall population. It has to be set against the vast silence of a wider world “that failed even to begin to share in the moderate amount of economic growth” that had placed our Harvester and many like him in the town halls of cities such as Mactar.
The Harvester mentioned in the last sentence was an anonymous farmer who did eventually join the local elite in the late Roman empire, but he was a rare exception. Most of his compatriots left behind no written record of their lives, so instead of people like him, we can only really talk about the lives of senators and emperors, rather than the vast majority of the population, whose voices are still drowned out by the philosophical babblings of 'civilised' men. These people had practically no political power and little legal recourse to defend themselves against oppression. Wealth still trickled down and political stability no doubt provided better lives for them, but it was still a fundamentally unjust and unequal society, which was no different to any society during the 'Dark Ages'.
Aside from economic impoverishment, we also have to take into account how brutal the ancient world was. Purges and massacres were a regular fixture of Roman history, just look at the conflict between Marius and Sulla, or the gruesome deaths of many populist tribunes of the plebs. The same was true during the imperial period, as the elite regularly engaged in conspiracies and purges. Though I read this years ago, I still remember a particularly chilling event from the reign of Tiberius when the family of a disgraced praetorian prefect was executed:
Then in late November or early December, though popular fury had by this time died down, the senate decreed the execution of Seianus’ two remaining children, Capito Aelianus and Junilla. Capito was old enough to know what was in store, but Junilla asked over and over again where she was being taken and what she had done wrong, promising not to be naughty again and begging to be let off with a whipping, as she was dragged to prison, there to be raped by the executioner – ancestral custom forbade the execution of a virgin – before she was strangled and her body exposed on the Gemoniae along with her brother’s.
Many of the 'barbarian' kings, such as the Merovingians who ruled post-Roman Gaul, were equally brutal and often murdered younger members of the family to end potential dynastic threats. Yet they can be very human as well. Clovis, the first Merovingian king of Gaul and who is famous for smashing his subordinate's head with an axe in public, also valued the advice of bishops and took care to treat with respect a Parisian holy woman. Clovis was no doubt a violent man, but did that make him an inferior to Augustus or Alexander the Great? I don't think so.
You might also be perhaps thinking of the great monumental architecture of antiquity, which had very few parallels in the 'Dark Ages'. This is I think fairly easy to explain - without a specific culture of civic building projects or a massive surplus of wealth, it is kinda difficult to build a new Parthenon or maintain a complex aqueduct system. To simplify a complicated academic debate, the people in the 'Dark Ages' (in most cases) did not have enough concentrated wealth to embark on grand building projects. Unsurprisingly, political fragmentation and economic decline tend to have that effect, but was it their fault or a characteristic of their times? Hardly, since this kind of thing happens all the time.
I think it would be useful here to quote the Greek historian Evagrius, who wrote at the end of the sixth century. He lived in Antioch and so was not living in a stereotypical part of 'Dark Age' Europe, yet his life was nevertheless filled with tragedies. Earthquakes had repeatedly struck the city, the Persians had sacked it in 540, and the region was continuously troubled by the plague. His words describing the last ailment is I think particularly poignant:
And often this [the plague] occurred to cities and other places in the rotations of the cycles that are called indictions. But an almost complete extermination struck mankind especially in the first or second year of the fifteen-year cycle. This indeed befell me, who has composed these things for I decided to interweave my own affairs also into the narrative, attaching appropriate matters where appropriate. And so at the outset of this great misfortune I was affected by what are called buboes while I was still attending the elementary teacher, but in the various subsequent visitations of these great misfortunes I lost many of my offspring and my wife and other relatives, and numerous servants and estate dwellers, as if the indictional cycles divided out the misfortunes for me. Thus as I write this, while in the 58th year of my life, not more than two years previously while for the fourth time now the misfortune struck Antioch, when the fourth cycle from its outset had elapsed, I lost a daughter and the son she had produced, quite apart from the earlier losses.
Evagrius had lived in grim times, but what struck me the most is that he still lived his life to the best of his ability. He had a family, he continued to work his estates, and his history-writing continued unabated. In a paradoxical sense, perhaps we should instead admire those who lived through such wretched times (if the 'Dark Ages' really were that bad). They certainly had to face more challenges than those who lived in more stable and productive times, yet their age was needlessly condemned. They were no different from more 'civilised' individuals living in better times, but because of their time and place, they had to spend more time fighting or planting their fields. People generally try to do the best they could in the circumstances, so condemning people who lived through events we can't even imagine seems to me to be utterly unempathetic. Instead, we should perhaps look at history more empathetically and try to understand their choices. They were no better or worse than you or me, just people who had to make different choices in very different circumstances.