Is cutting and self-mutilation a sin?
Under the Law of Moses it was a sin for Jews to cut themselves as a sign of mourning for the dead. "Ye shall not make any cuttings in your flesh for the dead, nor print any marks upon you: I am the LORD." (Leviticus 19:28) "Ye are the children of the LORD your God: ye shall not cut yourselves, nor make any baldness between your eyes for the dead." (Deuteronomy 14:1) These laws, though, applied only to cutting for the dead.
I suspect your question deals more with cutting for other reasons. There is nothing in the Bible to indicate that cutting is, in itself, a sin. It is a symptom of deeper mental health issues. It should be treated as an illness, or the symptom of an illness, and not as a sin. If you know of a person who is cutting or mutilating herself (or in rare cases, himself), you should try to get that person to see a doctor about it as soon as possible.