I always figured Cheryl works because she's not nothing better to do, without the job she's just sitting around in her mansion sniffing glue all day. Maybe it was a condition of her release from being institutionalized? ISIS gives her structure (a good thing considering how nuts she is), something to kill time with, and she gets to watch all the crazy shit that goes on there. Hell, a workplace like that, I'd hang around even if I didn't need the money. It's not like she's got her nose to the grindstone when she's there.
Anyway, I don't think the show has slipped too far. It's not as fresh and inspired as it was in the first two seasons, and yeah some of the jokes are a little repetitive, but overall I'd say it's about as good as you can expect an animated comedy to be in it's fourth season.