So a great book on the topic is Food in Medieval England: Diet and Nutrition edited by C. M. Woolgar, D. Serjeantson, T. Waldron.

The diet of peasants is, in large part, a bit of a guessing game. Literate people rarely wrote about the every day life of peasants but we do see many images of daily peasant life in artwork. We do not have any record written by a medieval peasant who wrote about what he ate everyday, though that would be super helpful. Also, time and location play a major part in this discussion. What a peasant ate in tenth century in France might not be the same thing eaten by peasants in fourteenth century Germany.

I am not a nutritionist or a dentist so my knowledge on that aspect of the subject will be limited.

The nobility would have access to rich and fatty foods and that is probably why Henry VIII was very fat by the time of his death. However, their meals were consistent and did not have to face starvation or malnutrition. They would have had access to a high protein diet throughout their childhood which would have aided in development. Their teeth would have been strong from just having enough food throughout their lives.

Peasants did not have consistent access to food. Their diets relied heavily on grain and preserved foods like salted or pickled foods. Relying heavily on grains means a lot of bread. The creation of flour required the grain to crushed using mill stones. The mill stones would have made the flower gritty because the part of the stone would erode into flour. This would have caused their teeth to grind down because they basically have sand in their bread. Plus, they most likely did not receive proper nutrition while growing up.

So, I️ would expect the nobility to have good teeth and the peasants to have worn down teeth. The sugar nobility has access to was not the refined sugar we use today. Though it was used it’s use wasn’t as heavy handed as it is today. Nobles most likely had the better teeth.