From what we've seen of her I'd say lesbian would be the most apt term. The whole identity involves a bit of gender-queerness since womanhood is kinda defined by a supposed subservience and thus expected attraction to men. So to be a woman who's exclusively into women is to kinda reject what being a "woman" is (hence why shit like stone butches who go on testosterone, nb lesbians, he/him lesbian, etc exist. People who's gender identity might be a bit fuzzy but their lesbian identity is very clear)... and to even keep this tangentially on topic, I think Momiji is a pretty good example of someone whom doesn't easily fit by most GNC/trans culture labels. Like she clearly was super into the idea of being able to become a guy in this chapter, but she also enjoyed expressing femme in her girl power gyaru makeover in a prior chapter. Most labels won't easily fit. I'm not even sure "genderfluid" is a fair label to impose on her either.
Momiji being into the idea of turning into a guy stems specifically from her desire to be with Mahiro, not a desire to be a guy. At the moment she might not be fully aware that two girls can just be together, even as adults (and even get married in some countries).