Rocky is not only one of the greatest sports movies ever, it is one of the greatest films ever. It was a dramatic, gritty, and surprisingly grounded film that redefined the genre, established now common tropes, and basically created the modern sports film. There's a reason it won best picture over Taxi Driver.
Rocky II took the tropes of the first film, pulled out the guts, and refined them into what we now know as a boxing film, turning the franchise into an enjoyable spectacle instead of a character-centered meditation on fame, toughness, and family. Rocky II does sports movie really well. There's no one who gets to the end of Rocky II and isn't holding their breathe as Apollo Creed and Rocky both try to get up off the mat. There is nothing wrong with Rocky II other than the fact that it isn't Rocky.
There is nothing wrong with Creed II other than the fact that it isn't Creed.