I have not had any but I've know a couple of people that have had kitties that didn't seem quite right. One such kitty is Etta J and she is the sweetest little all-black furbaby, but you can just tell by the look in her eyes that the litterbox is not quite full. She's very skiddish too and doesn't seem to clue in to things. For example, it took her months to remember where the food dish is. Meowmie would carry her over and place her in front of the dish, then the next day Meowmie would put the dish down but EJ would be walking around looking for it. Mental illness? Maybe but we just don't know.
I don't know about Munchkins but it wouldn't surprise me if the "challenges" they face are due to the breeding...and of course we would not change them. That's what makes them so special and lovable.
Life should NOT be a journey to the grave with the intention of arriving safely in an attractive and well preserved body, but rather to skid in sideways, champagne in one hand and strawberries in the other, body thoroughly used up, totally worn out, and screaming WOO HOO - What a Ride!
Sometimes the most real things in the world are the things we can't see
Until one has loved an animal, part of their soul remains unawakened.