I don't want my fiance to get fake eyelashes every month.
Why don't you want your fiance to get fake eyelashes?
It's too girly and I'm not into that
Let your fiance do her makeup as she pleases. If it makes her feel good, you should be okay with it.
I'm not sure how to disagree with something so seemingly harmless.
It is okay to not like it, but there is no reason to disagree with her about it verbally. It can create more problems than it is worth.