This satire of Hollywood hangers-on and wannnabes (not celebrities) could be depressing because it's so ugly; but since my physical and spiritual life are both thankfully distant from Hollywood and its noxious culture, I read it with detachment (clinical detachment, perhaps), and found the novel not particularly depressing, only unpleasant. Dead and desolate. The keynote scene for me, that which struck me emotionally more than any other and made an impression to me on behalf of the entire novel, is that in which Homer Simpson--the creepy and pathetic retired accountant,--sits outside his building watching a few flies buzz around a scraggly cactus patch.
I was just thinking that had this been a movie instead of a novel, it might fit very nicely with Network, the legendary satire of network news and arguably my favorite movie. (A Day of the Locust movie was made at some point, but it wasn't very successful.)