I'm not going to cheer on my friend.
Why? Are you angry with him?
He made fun of my weight.
That's really unfortunate that your friend made fun of you and it probably truly hurt your feelings. Rather than not cheering him on, you should have a conversation with him and let him know how his actions effected you.
I'm a jerk who doesn't care.
Don't stoop to his level, it won't make you feel better in the long term.