It's the incongruity of turning a Superman movie into a Richard Pryor comedy. It's not as incompetently made as Superman IV but it's infinitely more baffling.
Superman III was nonsense; the Salkinds knew they were watering down the property (more than already seen with the slapstick in II), all to jump on the popularity of Pryor, which--as the film clearly shows--was a greater priority than making a Superman film. Its only bright spot was the Smallville sequence, with the natural, believable chemistry Clark had with Lana, as opposed to his dysfunctional relationship with Lois. Lana should have been elevated to the love of his life status in the film, but that--ultimately--was smothered by the Pryor BS and all things Vaughn (a very strong actor in other parts).