I'm reminded of the George Carlin bit where he explains that every topic can be funny, and then he proceeds to tell the audience to imagine Elmer Fudd raping Porky Pigg. I agree with this, but I think some tragedies, particularly recent ones, can be an uphill battle for poking fun. It has a lot to do with context. Someone posted the little girl in front of Hitler making a speech, but what if she was in front of a haunting photo of a pile of dead Holocaust victims? You might not be offended, but would it be funny?
The Twin Towers blowing up might be kind of a touchy image for a lot of people, and it's kind of an obvious choice if you want to reference 9/11. What if instead of the towers, the girl was in front of that photo where the Secret Service agent whispered in Bush's ear that the WTC had been attacked? Might be a less offensive and more clever way of invoking 9/11. However, would that be funnier than the towers blowing up or not funny? Honestly, curious about what people think of that.
Regardless, I think the topic of "how to make tragedies be comedic" is an interesting one.