"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.
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.