I'm going to fat shame a girl who called me ugly
Have you ever heard of the term two wrongs don't make a right? Why do you want to fat shame a girl just because she was rude to you?
Because it was mean of her to do that and she deserves a taste of her own medicine.
I understand feeling frustrated and angry because she insulted you. Instead of insulting her, you should either calmly explain to her how you feel, or just ignore her. You shouldn't escalate the conflict by insulting her.
You're right. I'm just angry, but I'll just ignore her from now on.
That's a good choice! I know you're upset, but you're doing the right thing by being the better person and just ignoring her. You will feel better this way.