I'm going to make a joke with my ex-bf that goes a tiny bit too far
why do you want to make such a joke?
I appreciate her personality and I want to promote it by making a joke about her.
It's okay to get carried around when joking, but you shouldn't make fun of people just to entertain yourself. You may think it's  just a joke but they could become offended.
It's just a joke and also why shouldn't I make fun of her?
I get it if you got carried away with a joke but it's not ok to deliberately make fun of someone. He is not your boyfriend anymore but that doesn't mean you can offend him just to amuse yourself.