I hardly think the scene in the movie with people openly weeping and retching seeing footage from the bombings is part of the movie "glossing over" the human effects of the bombs. It's just not what the movie was about.

As for the general public, you can't just expect people to never have a sense of humor about something ever for all time, particularly when it's something that can occupy a significant and impactful sense of brain space.
It's how people relieve some of the emotional tension of a heavy topic. It's why we had COVID jokes and memes, and it's why in the past you saw a lot more nuke humor. There was an omnipresent specter of "there's a weapon that can kill everyone, it can kill us at any moment, we keep building more, and I'm utterly powerless in the face of this fact".

Laughing at the juxtaposition of Oppenheimer and the aesthetic that barbie presents requires an understanding of the horror of what the man ultimately produced.