Maurice is an angry pimp who prostitutes out a young girl, steals five dollars, and punches Holden in the stomach. He wears a "phony shirt collar" under his uniform, has a "big fat hairy stomach" and looks "very, very tired or very, very bored" the whole time he's intimidating Holden into paying five more dollars for the privilege of talking to a nervous teenager (14.20, 22).
And Holden misses him.
So, what's going on?
Well, the whole reason Holden agrees to the prostitute thing in the first place is that he's "depressed" and isolated. In a way, it makes sense that he thinks he "misses" all the people he met over the course of the novel—even Maurice. The world may be full of phonies, morons, and bastards, but he needs to connect with them anyway—even if he's connecting with a hard punch to the stomach.
The whole incident with Maurice also helps us see that Holden just can't help making things worse for himself. This isn't the only instance of Holden intentionally making things worse for himself. You could (very, very easily) argue that he purposefully gets kicked out of Pencey, drinks to dangerous levels, and wanders around the freezing park at night because he's acting out a wish to self-destruct.
When you put it like that, trying to get punched by a "pimply" elevator man doesn't seem too bad, does it?