I would never, in my life, tell a 9/11 joke. They always crash and burn.
Is this a joke in it of itself?
Yes, it's a clever little joke. I think it's rather funny.
While the idea of you making this joke was to get a laugh, it is still very insensitive to joke about something like 9/11. Just be careful to not offend people with jokes like this because 9/11 was a huge tragedy in the US.