If it's a play on 'ok boomer' one would expect it to be less offensive. 'Groomer' is categorically a terrible thing to be, often borderline criminal (if not outright) with implications of sexual abuse. Boomer, on the other hand, is more exasperation than dislike (after all, boomers are our parents/grandparents, they're hardly a group we're going to truly condemn in our personal lives).