I don't think JJ is mediocre at all. While I didn't care for 'Into Darkness' I liked his first Star Trek. Keep in mind, he has only directed 4 movies, and a few episodes of his shows. I do think fanboys tend to go 'Strike one, you're out' with certain directors or actors, which is invalid.
I don't think he's an auteur, but I think he's a fully capable director who is very good with actors; and mixing practical effects with CGI. To me, he's better than Bay, Snyder, and a few of the popular directors out there.
I think he's a good director who could be GREAT. I think he'll hit his full potential with Star Wars 7.