Writing level: the noticeable quality difference (which can't be helped by the actors) is a key part of the characterization and plotting.
Which isn't to say that George isn't still a top tier dancer! The gap is waaaay smaller than, say, the Sinatra/Kelly team-ups. George, after all, did pair with Eleanor (and Buddy) as the actual leading man in 1938.
This is still true to the characterization and plotting in this movie, as King is able to execute to the satisfaction of anyone who hasn't seen Johnny yet.
At the 25th Academy Awards in 1953: Bobby Driscoll, Dean Jagger, Ginger Rogers, Ray Milland, Olivia de Havilland, Charles Coburn, Broderick Crawford, and George Murphy #Oscars