Hey, I got a joke for you: What's the difference between the Twin Towers and genders? There used to be two of them, but now it's considered rude to talk about it.
Are you saying it's not okay to joke about the twin towers or genders?
It's a joke okay. Why do you have to take things so seriously?
9/11 was an event that shook American citizens to the core and which many people lost their lives. The families of those victims continue to suffer and feel the impact of those events. Drawing focus to such an event only to joke about it can be seen as cruel and uncaring.
So you think it's still too soon to joke about 9/11?
You bring up a good point about humor and distance from an event. When somebody falls ass-over-teakettle, the difference between comedy and tragedy is how well you know the person. In this case, there's still thousands of people whose lives were tragically altered that day, so in answer to your question: yes, probably, especially for people over 35-40 years old.