Self-mutilation is when someone hurts themselves to release some form of pain. Usually cutting, pulling hair, burning, banging etc. Making fun of them is not going to help. Just because you have different ways of coping doesn't mean you should make fun of people that already have problems hating themselves etc. Self-mutilators need help they don't need people picking on them.
Self-mutilation is dangerous. If someone you know is self-mutilating get some kind of help.