My answer concerns the later Middle Ages. Hopefully one of our early medieval experts will be along to discuss Chris Wickham's thesis about peasant life in the earlier period.

This is kind of an impossible question to answer because, as Paul Freedman points out, we have no unfiltered peasant voices from medieval Europe. Archaeologists have uncovered massive amounts of material evidence, but anything textual is filtered, selected, and pre-interpreted by the upper classes. Steven Justice has argued that six little texts (in the form of letters, although he thinks that at least one was originally a sermon) that survive in two late medieval English chronicles represent the viewpoint of the Peasants' Revolt of 1381, but notes that the "peasant" part of the title is to some extent a misnomer.

For the most part, we have court testimony. This can include peasants suing other peasants. More relevant to this question, it includes a LOT of peasants protesting oppressive conditions. Thomas Bisson is probably the scholar who has pushed this line of inquiry the farthest, stressing the frequency with which the word tyrannus appears all over eleventh and twelfth century sources in particular. In Catalonia, destruction and seizure of peasant property was how knights built themselves into lords. Lords in Germany (and almost certainly elsewhere) oppressed their own peasants, restricting access to hunting, foresting, and fishing areas.

In the sources, we see two patterns: peasants protesting their own lords' oppression, and peasants protesting their lords' failure to protect them from marauding other lords. Peasant communities were frequent victims in the crossfire (literally fire; arson was a big concern) of princes jockeying for land and power.

So with the words tyrant and tyranny flying around the sources; peasants summoning up their courage to argue their case before their lords; even Church councils censuring their own prelates for being naughty ecclesiastical lords (usually the Church just censures the laity), surely we must accept that medieval peasants saw their lords as a collective terror!

Except...this is where the source bias comes in. Peasant protests are recorded in the sources because they were formal petitions. There will be no record of "thank you notes" from grateful peasants, given their illiteracy and the cost of hiring a scribe plus obtaining writing materials, plus the unlikelihood that preservation of such letters would have enhanced the status of the receiver. What we do see, in the late Middle Ages, are attempts on the parts of lords and kings to mitigate the effects of famine--not only for their own economic gain, but also for the protection of their peasants. During the 1315-22 Great Famine, for example, English nobles tried to implement price fixing on grain crops, even though it might have benefited themselves to sell what yield they could at high prices.

The late Middle Ages were rocked by a series of peasant uprisings for sure--the 1381 PR, the Jacquerie in France, the Shepherds' Crusade (which David Nirenberg argues is frustration at lords turned onto outsiders). The interesting thing with a lot of these, though, is that peasant demands tended to be linked to very specific measures: a new poll tax perceived as unfair, the fixation of wages for day labor at levels the rebels considered ridiculously low and old-fashioned, new restrictions on water rights. Do these revolts represent a generally bad situation that occasionally got really bad? Or a tolerable world where sometimes lords pushed too hard?