Ugly disses are as old as 'your mama' jokes. There are millions and they never go out of style. "You're so ugly, when you were born your parents didn't know to put you in a crib or a cage." Or, "Your so ugly, when your mom gave birth to you she pushed you back in." That was brutal.
"I'm not saying you're ugly. I'm just saying if you go to the veterinarian no one will ask any questions." These are usually very funny unless you get a more serious one, like, "Ugly girl, why you playing hard to get, you're already hard to want." Ultra brutal. If you are actually ugly, you could do as this diss says, "Don't think of yourself as an ugly person, think of yourself as a beautiful monkey."