That was a tragedy in US history. Some people can move past tragedies and are able to joke about them. Some don't. Notice I didn't say "can't".
I'll bet you're sleep won't be affected by his joke. It was pretty mild. One of two things: You're either making more of it than you really feel or you really are a very sensitive conservative. And I wasn't yet an adult at the time but I do remember it. Would that qualify me to joke about it?
The anniversary of the event was yesterday, so it stirred up memories of those days. A glib remark may have been innocent enough. I don't know.
As to moving past things....we all carry the rocks and scraps of our past life with us until we die. Those who say they don't are lying to themselves.