I've been accused many times of trying to make light of a bad situation, so I know where you're coming from. But that was a profoundly disturbing day for me, and it wasn't until the Tourist Guy photo made its rounds that I could begin to think of it in less than completely serious terms.
This particular "art sign" doesn't bother me personally at all. I see the humor. But I think it's needlessly hurtful to people who will never see the humor, and for that I reason I think it was not a good idea.
Well, it is insensitive but a lot of humor is hurtful. It's just expressed in a context that is acceptable. Sometimes people cross the line. I lived in nyc about 1/3 mile from the former WTC, and amidst the atrocity, I still found stuff like Gilbert Godfried's 'I need a direct flight from NYC to LA' joke very very funny.