Socially I feel like the vibe right now is that it's normal/trendy for guys to "like ass", but at which point does it become too much?
if (during sex) a guy presses his whole face between my cheeks am I supposed to think this is completely normal part of sex or am I supposed to tell him that it's kind of weird? (I don't even know if it is normal or not)
“Too much” is definitely subjective. It’s whatever YOU feel uncomfortable with. If you don’t like something he does, tell him right away - “Don’t do that.” Or “I don’t like that, don’t do it again.” Say it firmly, with confidence, and then move on right away. Don’t let him argue with you about it.
For me, I feel if a man is pushy in any way about me fulfilling his preferences after I have told him I don’t enjoy something that he wants, that is where I draw the “too much” line. I also would draw it immediately if he is trying to discuss explicit sexual activities before we’ve gotten to a point of that being a comfortable conversation for us. For example, men who you are in the talking stage with who start asking you for anal sex, or talking about wanting anal play of any kind, I simply won’t continue that conversation because it’s clear that if I say no to that, he’ll either lose interest (showing me that he’s interested in my body and not me as a person) or he will try to push my boundary, which is unacceptable.
Always, always, do what is best for YOU. If what he wants is not in alignment with that, leave him alone.