The idea is unnecessary surgeries, including tattoos. Zit-popping and scab-pulling, if harmful activities, would probably be included here. However, if they do produce harm, it is (very probably) too minor so as to constitute grave matter, therefore something you do not need to confess. The fact that you will probably not know the exact status of harm or non-harm unless you are a skin-doctor would mean you have not knowingly sinned. I suppose that if you had infinite time, you could research the matter more fully, but it would be an extremely low priority. Besides full-knowledge considerations, I would definitely bet most scab-pickers, etc. don’t have full consent of the will (it itches). Basically an 0-for-3 on the need-I-confess-it list, and quite possibly not sinful in the first place.
Back to meanings of self-mutilation, generally. There are several tattoo threads on here for you to browse. Probably prudence (weighing the risk of harm vs. their upsides) is the yardstick to measure them by since it is a tough sell to say they are always objectively sinful. Vasectomies are indeed very serious sins of self-mutilation. Cosmetic implants probably follows the prudence yardstick as well, and there might be an underlying element of vanity (same with tattoos) that would bear close scrutiny as well. Self-flagellation can be a very holy penance if done correctly, but might even be sinful if not. Good use usually requires the advice of a holy spiritual director.
Probably the more common forms are drug use with any dirty needles being extra bad.
I hope this helps while not being too-over-the top. God bless!