Thomas Aquinas offered three points to consider regarding the morality of an action:
1) The act itself
2) The situation
3) The motivation
Seeing scores of people waving flags, cheering, shouting "U-S-A!" I've no idea their motivation or intentions, but it does remind me at times of those who stand outside of prisons and cheer as people are executed.
Not an exact parallel, as this might be more in line with the famous photos from V-J Day. I doubt those people were cheering because of what happened in Hiroshima and Nagasaki.
But I don't know. That's the only conclusion I've come to. I can't stare into their hearts and determine why they're happy.