In general, for medical procedures, tipping is not usually customary. Also, with any medical procedure, whether it is botox or peels or laser, these should be done by a physician, not estheticians or nures practitioners.
If an aesthetician does a superficial peel, which is an exfoliation, than you can decide whether to tip them or not. You should not have an aesthetician do Botox injections on you as that is a procedure that should only be done by a licensed provider.