no, I think it's a twist on a remark made by George Bernard Shaw? to a Lady Something or other. When she said "Mr Shaw, you're drunk!" , he replied, "yes, madam, and you are stupid, but I will be sober in the morning..." or something like that...
Ugly and slimmer aren't connected, it's just that if one is fat, one can do something about it, whereas if one is the sort of person who shouts random nonsense at people, one is probably doomed for good...
I'd rather call someone in the situation thick, I think, than ugly, as i'm not sure I've got much to crow about in the looks department, but I do know I'm fairly bright...