I'm going to joke about someone's weight
Why would you joke about someone's weight?
I just want to make her miserable.
You will hurt her by making fun of her weight. It's not right what you're planning on doing. You should be ashamed of yourself for judging people by their weight.
That's fine by me. Besides, her weight is terrible, it's funny enough for me to cry about it.
People have many reasons to be overweight and by making fun of them you could be causing to eat more and gain more weight. You should stop yourself from jokes about, you would not like it if people joked about things that you were insecure about.