There’s nothing inherently wrong with Top!Sam Wilson in fic.
What’s wrong is that he’s portrayed 98% of the time in a very gross way that is steeped in “Big Strong Black Man” who’s aggressive, at least sexually, and his sole concern is providing comfort for his delicate white boyfriend woobie.
Typically, he proclaims that his new purpose in life is to take care of the white guy, find what makes him happy (more often than not ANOTHER white guy), and generally just shoulders all of his problems without any reprieve.
And don’t get me started on the connotations of the “Big Black Cock” issue that’s actually rampant in real life LGBT+ communities, along with general antiblackness. Like, the author seems to think it’d be ~sexy~ for the blushing white guy to be ~ravaged~ by the strong, aggressive black man.
It shows that fandom is incapable of viewing black characters as sensitive, gentle, worthy of being on the receiving end of affection, etc. And that in the aforementioned case, Sam’s sole purpose is to ‘spice up’ Steve’s sex life and hold him over until Bucky inevitably returns.