Comedy is an important coping mechanism. There should be NO topic which is beyond comedy. I've heard jokes about the Holocaust told be Jews which are hilarious. If you have no taste for gallows humour that's fine, it just means you are not the intended audience.
Watching comedians talk about this very topic, the main thing seems to be if you're gonna go there, it better be funny. Just doing a topical joke on some recent disaster for being topical's sake is in poor taste.
One standup said you know it's a good joke on a touchy subject when the audience is appalled but can't help themselves laughing. The whole crowd cover their mouths with their hands.
As for 9/11 and the Holocaust specifically, sorry Red, but it's really not too soon. In fact, specifically for 9/11, yes it was bad by the standard of what we experience in the fairly cosy first world, but an equivalent act in Palestine doesn't even make the news. For how many years was even suggesting anything other than "it's the worst things to ever happen anywhere ever" tantamount to unpatriotic treason? If not for humour blazing the trail, serious discourse about it would have been harder if not impossible.
Everyone copes differently. I've had three parents die of cancer. It sucks. But I make jokes about cancer when a good opportunity arises, because that's how I overcome tragedy. Do I have the right to do that?