I'm going to make a joke about my friend's mom
What is the reason for making fun of their mom?
Her mom is fat and ugly and I hate her.
It's not nice to say those things about anyone, but especially about your friend's mom. That's horrible to say those things about her.
It's just a joke.
Joke or not, I think that the words are still hurtful. You should not make mean jokes about a friend's mom.