I must have missed the part where these memes are making jokes about the bombings of Hiroshima and Nagasaki.

Simple: It doesn't.

Barbenheimer, also known as Barbie Heimer, is a portmanteau of the words "Barbie" and "Oppenheimer" in reference to the 2023 movies Barbie and Oppenheimer, which were released on the same date (July 21st, 2023), causing a slew of Barbie vs. Oppenheimer memes that humorously combined the contrasting aesthetics and themes of both films. In turn, the word "Barbenheimer" spawned on social media and gained its own traction in memes and viral discourse, mostly in reference to the act of doing a double feature of both films. Many Barbenheimer memes and fan art used images that were both pink and black (correlating with the color palettes of both films), as well as images that involved both explosions and girly aesthetics.

It's just a typical case of taking a thing and getting riled up about it, without making a simple Google search, what the thing you don't like is even about.

Seeing they made 9/11 memes to retaliate against Americans already, why not keep barbenheimer? Tea is already spilled on both ends, everybody is unhappy, that's a compromise for ya