Monica, I could not disagree with you more. In the book, Jadis is described as being "dazzlingly beautiful." Someone can look beautiful and a the same time evil (actually it said the Charnians looked more and more cruel and proud - and that Jadis was the cruellest; no mention of 'ugliness' is made). Lewis was in fact counteracting the Medieval idea you mentioned by having a wicked character also being beautiful (whilst at the same time still appearing 'evil').
Also, this idea about society getting worse and worse ('going to the dogs') is a common human feeling that we all share, which although being entirely natural, it is not, I suspect entirely reliable or accurate. Just to give one quick example (there are many more), slavery, two hundred years ago, was commonplace and acceptable, and now it is against international law. Wouldn't you say that this is an improvement? I am by no means a 'progressive' but we must admit that many, many things have got better, more civilized, more humane. I suspect when many people refer to the 'olden days' as being morally superior to nowadays, that they are thinking of the 1950's, when admittedly, people had higher moral standards than today, but if you go futher back you will find standards getting worse and worse. Even in the 1950's or 40's though, racial and sexual bigotry was much worse than it is now.