In reality, this is subjective of course. Medieval peasants had the potential to be about as smart or as dumb, as nice or as cruel as people from any other time or place. Thing is that beside our accumulated knowledge of age and literate population, vast amount of easy accessible information, intelligence has actually evolved with each generation. Not only in the sense of better nutrition, medicine and quality of life, but since the advent of the scientific method, the way people react to the world has shifted in ways that would seem impossible or insane for ancient societies.
In other words, our cultures are better suited for comprehension, studying and analytical thinking.
On the other hand, the general conditions of living, while not nearly as appalling as they're made out to be, were considerably more demanding, and just surviving required not only more physical but also more mental readiness than the Medieval era is given credit for. Conversely, while we certainly have access to more information nowadays, most people don't exactly take full advantage of this, and even then, we have to keep in mind that "knowledge" is not synonymous with "intelligence". If anything, studies tend to show that being saturated with information faster than you can process it can actually impair cognitive function.
As for the studies showing that intelligence evolved with each generation, they should be taken with a grain of salt. Not only do they only go back a century and half at most, but they're also extremely culturally specific: these are the same studies that show Europeans consistently outsmarting Africans. What we see as intelligence in our own, modern, Western society is a very biased view: it would be downright moronic to apply most of our behaviors, mores and even thought processes in most situations outside our own incredibly complex and specific society, while a lot of our knowledge is not only completely useless but also completely unverifiable without the vast social and technological infrastructure we have. There's a reason why "normal" behavior being completely useless in a survival situation is such a big point in military training.
This problem arouses the ancient question... Who would win: Astronaut vs Caveman? Each has a fierce Fandom who will fight tooth and nail for their choice. While history had shown that having more "advanced" society and better weaponry/gadgets doesn't mean you are invincible or cunning, the problem is that when people think of Peasant/Medieval/Underdeveloped, they instinctively give them the label of Middle Class/Average Joe and everyone knows that the Underdogs Never Lose.
Also remember that the great people of the past are exceptions to the rule, extremely rare and bound by the very society and situation they live in. Using them as examples is not that relevant.
While of course, most people today think they know everything about the universe (just like back then), naturally peasants always had great practical expertise in areas few modern European or North American humans would bother to learn any more. This led to Crippling Overspecialization in a single area that would not make them very flexible thinkers or cunning per se. Intelligence isn't necessarily knowing everything at all times and cunning doesn't mean being a chessmaster. This works both ways.
Finally, since the question of religion will crop up sooner or later: the unwillingness of people to change their situation was somewhat justified by their faith. The argument would've gone like this: Our life on Earth takes 70 to 80 years max (at least today it does note )-but our afterlife is forever. Hence it makes sense to care more about living according to your religion than trying to change your fate in this very world (and even so, what would/could you change in 50-60 years? Many horrendous movements and ideologies were started out of a desire to change the world which either didn't need changing, or was changing just fine in its own pace).
Further adding to the complex situation is the fact that every generation/age is subjected to its own type of a pity it feels to those "poor, unenlightened wretched simpletons of yore". Although we like to ponder how better things would be today if only X was done/invented/didn't exist, the fact remains that for the people living in those periods, life was probably reasonably good. Just as a medieval peasant could not pine for the internet, or lament the lack of democracy, so we today cannot desire some fantastic things from 3000 AD.
Also, it's important to remember that history as we understand it is not objective fact but merely our understanding of the past, and is subject to change as new sources are discovered or biases are discussed. We must remember that virtually every peasant was illiterate in the Middle Ages. In fact up until the introduction of cheap and reasonably easy to produce paper in Europe in around 1300 AD literacy was extremely rare and usually only found amongst the clergy. This wasn't so much a case of people being too dumb as simply the lack of proper writing medium - papyrus preserves very poorly in the wetter and colder European climates, clay tablets are bulky and fragile, and parchment is bloody expensive. Birch bark is a useful writing medium, but it was usually only found in abundance in Scandinavia and the Kievan Rus', which had higher numbers of literate people. This means that for centuries entire generations never got to tell their side of the story, and most of our understanding of the Middle Ages as a horrible time is skewed by the writings of Renaissance or Enlightenment writers pining for the days of Ancient Rome and Greece and looking down upon the "dumb" generations in between them, in the same way younger generations look down on Boomers as the "old and out of touch" generation.
