(That's right, I stole the title for this manifesto from a manifesto by Steve Martin. You don't have a problem with that, do ya, Steve? No? Okay!)
I don't think any subject should be off-limits for humor. (Okay, so maybe the title of this should be "What I DON'T Believe." Phooey. Life is hard!) The subjects of jokes, the things we laugh about, are often pretty horrible things. Freud wrote an excellent book analyzing just exactly what jokes are all about. A really terrific book. With good jokes in it. Laughing at a joke about something bad doesn't mean we don't think that thing is bad. Richard Pryor made great jokes about extremely painful things -- like when he accidentally set himself on fire and almost burned to death, hello, that was painful.
Good comedy heals. Cheap humor can just be crude and mean, but good comedy opens people up. It makes them MORE sensitive. Often it gets them to talk about things that need to be discussed, that they couldn't bear to talk about before. If my joke only made you hurt worse, then I failed as far as you as an audience are concerned.
I saw an excellent TV show recently with Ricky Gervais, Jerry Seinfeld, Chris Rock and Louis CK talking about such things. The title of the show was something like Talking Funny They all agreed that no subject is off-limits for comedy, although it may be very difficult to be funny about horrible things. But precisely that is one of the primary challenges of good comedy.