He can land an uppercut like one https://streamable.com/i0eel This "ponderous unathletic" Carnera myth is BS that has recently been debunked. Benny Leonard called his defense marvelous. He was no more ponderous than Vitali.
Carnera isn't nearly as bad as he's made out, but he was still very flawed in a number of areas, and his athletic gifts did not translate well into the boxing ring. You can see that in the way he shadow boxes and fights, very stiff unnatural movements and a very amateurish habit of keeping his chin in the air. That could have been a result of bad coaching, but he was what he was, and what he was doesn't approach what we could today consider a top flight super heavyweight.