I just finished Season 2 last night, so I'm admittedly several episodes behind where all of the characters are now....
Josh's lack of intellectual and emotional depth reads, at least to me, as a lack of character depth. Even his sidekick Hector, with his weird relationship with his mom, seems to have more to him as a person than Josh the personality-free sunny California dude-bro does. (Though I suppose him being a blank canvas Rebecca can paint whatever fantasy she wants on is part of the point.)
With that said, I think that the reason Josh bothers me isn't simply because of the character himself -- although that's a big part of it. It's because of the number of times he's been shoved back onto Rebecca's romantic radar as an option after she's supposed to be 'over' him -- to the point that I no longer found any tension in their will they/won't they anymore, and could summon up exactly zero enthusiasm whatsoever for their latest stint together. I much preferred the focus on Rebecca's relationships with her female friends and if they're going to fling yet another man at her, I'd at least want him and their relationship to be interesting -- which I find Greg, as well as the prospect of Nathaniel, even if they're toxic.
My reaction to Josh is "You are boring and your plots are boring and I would like you to go away because every other person on this show is more entertaining than you.