I would never joke about 9/11 to someone who actually lost friends or family during the event. On an individual level, it really was an unspeakable tragedy and immensely traumatizing for thousands of people.
But meanwhile the US government will still not apologize for nuking Japan twice, pulverizing primary schools and killing 30x as many people in the blink of an eye. So yeah, on that scale 9/11 is a hilarious joke indeed.