No selling one headbutt isn't the most impressive thing in the whole universe. It's not like it was his most powerful attack.
The severity with which she no-sold it was the problem, though. They could have at least had her flinch, but she didn't even react at all. It was a pretty blatant "OHHH" bait moment, but it turned out just to be super insulting to Thanos (and everybody who has thus far struggled to fight him).
Super insulting? C'mon man lol
Was it super insulting when Hela *Thor Ragnarok spoilers* shattered Mjolnir with one hand?
Yes it's supposed to be an oh shit moment. Oh shit moments are fun and the MCU is filled with them.
That wasn't super insulting because Hela was established to be the hammer's original owner. My point is that Thanos has basically cut down every hero who challenged him like they were wheat in a field, then Captain Marvel shows up with no investment in the conflict whatsoever and stares Thanos down with a "narrow-eyed smoldering cocky action girl look" while he gives her a headbutt and she just brushes him off like he's another random villain. It was dumb and shouldn't have been in the movie. ---