It's a stereotype, but then the comic might as well have been SGT FURY AND HIS ASSORTED STEREOTYPES. I imagine Lee and Kirby got this from those post-WW II movies with the squads that always had a Jew, an Italian, an Irishman, all interacting with just enough conflict. I don't recall those movies going so far as to put a black soldier in a combat unit, but in the Civil Rights era of 1963 it would feel right.