Yes, we understand that, but that's not the joke. The joke was not "eww fat naked people". The joke was the absurdity of the fact that Judy is shocked at seeing naked animals to begin with.
Because in reality, animals are naked all the time. It's not something that would be funny to the characters, but it's funny for the audience because for us it's not shocking to see an animal without any clothes on, so the concept of someone reacting that way to naked animals is inherently silly.