That's probably why the movie was titled Oppenheimer and not Hiroshima & Nagasaki, because the movie was focused on the man and his work and the regrets that came from that work, while nations celebrated the end of a second World War.
This movie was the type of movie that was always going to upset someone, and while it sucks that Japanese citizens were killed, their Emperor's military might was a brutally murderous raping scourge set loose on that section of the world, while also working with some of the other worst regimes of the world. Overall Japan fucked around and attacked first, did a lot of horrible shit to many different peoples, made some truly horrible friends, and then found out in one of the most devastating ways possible, I feel bad for the innocent civilians, but it was always ever going to end the way it did, if not a lot worse.
I'm just glad Japan grew to be what it is now and that it chose better ways to engage with the world than more attempts at domination, even though Anime, Manga, video games, and more have dominated the world's hearts.