Since 1990 there has been an epidemic of humans getting overweight in most of the world, as people have become fans of unhealthy cheap diets. US included. Most zooporn is home made by US citizens. So, many of them are overweight. That is one reason why so many zoos are fat.
Another reason why zoos are a bit more ugly than your average non-zoo, is that we do not care about getting a haircut or looking well as much as an average non-zoo does, as we are not looking to look decent for a human as to attract a human mate. We already have an animal partner and they don't care if I didn't cut may hair of if I wear ugly clothing or if I didn't took a bath today.
I'm also decent looking. I'm what is know in the gay community as Twink. And similar to Twinkies (the food that never rots) I look 18 despite being 37+
I would look way better if I wanted to, I could be well toned if I did exercise, have a nice hair cut and nice clothing, not have a tiny belly fat flap, have a 6 pack, etc. But I don't do that stuff, I don't try to look as good as I can, again, I don't care to get a human mate. Because of me not keeping my appearance in good shape I look a bit less sexy than I can actually look.
Also, I'm sure there are many good looking zoos. But they may have tatoos that can be easily recognized. Or they are too paranoid to share pics and videos of their bodies.