Elvis made a few good movies, many fun movies and a few bad movies. When a movie isn't good, then there's a whole range of entertainment before it can be called unwatchable.
Because it was the King, people didn't accept it that he played in simple, fun movies, while the whole musicscene (and world) was exploding.
Just imagine this: those Elvismovies without Elvis, but with a famous 60s Disney actor. By now those movies would be released by Disney on the 'Disney Classics' label and everyone would watch them with a 'aahh.... those were the days' feelgood feeling.