Basketball Reference says that he's shooting 39% from 16-23 feet this year, which is actually better than he 36% he shot from that range last year and his career average of 38% from that distance.
That's not "horrible" for long twos (for comparison, Kobe is a career 40% shooter from that distance, Lebron is 39%), but when you're:
a) A shooting guard
b) Don't really do anything other than score
c) Can't hit threes
It seems to me to be mediocre as fukk to be only hitting 38-39% of your long twos.
That, I think, is why Derozan is so poor in the playoffs. The lane closes up and a guy like him can't get into the key as much, and he doesn't have the long twos or threes to fall back on, so what can he really do? He just becomes a streak shooter who will go 4-15 more often than he'll go 10-20, and otherwise he ain't helping you very much.