Americans are also known for being really direct (YMMV; southerners are a lot more indirect, where West Coasters would much rather that you just tell them to get fucked than pretend that you like them). OTOH, from my experience hosting students from East Asia, they tend to be at the opposite end of directness. It took us half the school year to get the kid from Hong Kong to open up, start talking and joking without being prompted and be comfortable telling us mildly uncomfortable things (like if he was unhappy about something). We've had two Japanese students as well, and it's always been a constant struggle to get them to tell us if there's a problem; they prefer to reach out to a program supervisor instead of telling us directly, and one of them was so bad about indirectness that it felt like he was constantly lying to us. It turns out that differences in directness cause a lot more issues than I would have guessed.
I've heard from multiple expats living in Japan that Japanese people just do not recognize sarcasm at all and just react as if the person is completely serious.
America is a big place. Sarcasm isn't popular everywhere. Here in the rustbelt it's king. Other than one 9/11 meme I saw yesterday I'm not too aware of any 9/11 jokes, can say the same for Hiroshima & Nagasaki
The situation is similar in Korea. A friend and I went to see Drag Me to Hell here and we were a bit embarrassed to be busting a gut in a dead silent theater. Horror comedy doesn't work here at all.
Clearly, the Americans haven’t met the British or Australians yet when it comes to sarcasm. Or was this just another American touting they are best at everything again?
Hiroshima and Nagasaki had 60 times as many deaths and many more with lasting injuries including radiation sickness, and rather than being shoved down everyone's throats by the media and politicians as a justification to start a bunch of pointless wars that got a bunch of people killed, they were instead used as a reason to end a stupid pointless war that got a bunch of people killed. There's good reasons to mock 9/11, but not the atomic bombings.
More generally though Americans tend to be more irreverent, and imo a lot of that has to do with religion. Most Japanese people aren't religious at all, and their religions don't often intersect with the political sphere - it's almost unimaginable if all you've known is the US. In the US, mocking certain topics is a way of establishing which group you belong to, and there's many different groups in the US who all hate each other and are constantly making fun of each other.