I don't have a problem with her butt being visible at all. Like you said, people have butts and we see them everyday. I could tell this booty was emphasized though which made it just sorta feel a little comedic yet a little disappointing because Gamora essentially got reduced to a butt with a green woman attached to it. That bordered on bad taste to me because Gamora is more than that. It didn't bother me as much as the "shocker" panel which made me wish these particular characters weren't so appealing to kids. It was funny, especially given Carol's expression, but still kinda bothered the parent in me.
Edit: auto-correct sucks mightily!