I don't think it is inherently sexist to suggest that men or woman may exhibit certain behaviors more than the other. I am not sure when we crossed the line that making any such suggestion is inherently sexist.
"Mentrification" suggests that men enter women's hobbies and ruin them. It also suggests that there is a right and wrong way to engage with things (See conversations about how men and women engage with fiction differently, analysis vs elaboration, power rankings vs fan-fiction, etc).
Rather than saying the word "mentrification" is sexist, which is a complex question (like nobody is being hurt because someone on twitter comes up with a funny phrase), I'd rather say, if someone used mentrification in earnest, they're probably a shitty person and I don't want to hang out with them.
I agree that Mentrification is not the right word. The OP was implying it can happen in hobbies already dominated by men, not just hobbies dominated by woman. Perhaps Brotrification would be better.
It's pretty sexist to imply that only men exhibit this particular form of gatekeeping or making men in the minority of some hobby group feel out of place, and then coining a term for it with "men" right in the name, as though women never do what is described.
Complain about some subset of the population who has made a lifestyle choice you don't like (bros) all you want. But "mentrification"? Give me a break. That's just a prejudicial slur.
