This is a really subjective assessment rooted in a modern aesthetic. You might want to check the FAQ section for Why wasn't ancient art realistic?

There's also this question that got a lot of responses. Check out /u/hukgrackmountain's top answer and /u/GeorgiusFlorentius's response to it. Why are old drawings of people (1300 and earlier) not very good or lifelike?

If you'd like some more discussion of these issues, it would be helpful if you could point to some specific features of medieval art that you think are ugly or that simply differ from you're used to.