Should the nuclear bombing of japan in WW2 be considered a war crime?

No. Clearly. The American alternative was to continue Island hopping and absorbing massive casualties themselves on each island, while The Japanese including civilians often chose suicide before surrender, with footage of mothers throwing infants from cliffs then jumping themselves.
Yes. The populations of Hiroshima were sacrificed in a brutal and horrendous way. But that led to the surrender that saved the bulk of the population , plus countless thousands of Americans.
President Truman would have been run out of town if he had taken the other non nuclear option and it came out he had those bombs and didnt use them.