Targeting civilians is always a dirty and cheap strategy. The USA could have targeted military areas, government buildings, intelligence centres, etc. But instead they dropped the most destructive bomb of the time on Japans largest in-tact city (Tokyo was already majority ruble).
The bombs killed 90,000–146,000 people in Hiroshima and 39,000–80,000 in Nagasaki. The majority of that being innocent men, women and children. Yes, there was a large portion of hostile enemies in that mix too. The argument at the time was that there is no point bombing Tokyo as it was already flattened mostly. The bomb would be wasted there. Also, the Japanese manufactured a lot of arms in their homes, so there weren't many ways to bomb Japan with a single bomb and take out lots of military infrastructure. So Hiroshima was the obvious target.
All of the reasons make sense, and half justify it. But only when you're thinking "how do I use my massive bomb". There were other ways to go about it. I'm undecided with this one. I think it really illustrates the horrificness of war, and that the innocent country defending itself is still a monster. No good ever comes from war, and there is never justice in killing. At the end of the day, hundreds of thousands of innocent people died. A lot of them bystanders in the war, and not enemies. That blood is on America's hands.
For the record, I think it made strategic sense, desperate times and desperate measures, etc. But I really don't know if it was the right decision or not, I'm undecided.