The US is not a single entity. It's an extremely heterogeneous country composed of 300 million people, who you can divide into a variety of sub-demographics based on age, ethnicity, race, class, religion, ideology, and so on. So one should not expect a single answer to this question, either today or in the past.

If you are asking about today, that puts us beyond this subreddit's remit (20 year rule), but a YouGov poll from last year (the 70th anniversary of the bombings) surveying American attitudes towards the bombing (question #4). If you take a look at it, you can see that it breaks down along a variety of lines. Age in particular matters a lot: older Americans tend to see the bombings as justified, younger ones do not. Whites are largely in favor of it, other races not so much. On the whole, slightly under 50% of Americans polled thought it was the right decision to make, and the other half were pretty evenly split between thinking it was wrong or not being sure. I might also point out that question #1 indicates to me that Americans' understanding of what happened during World War II is a little shaky on even basic facts (fewer than 50% of Americans under 44 could correctly identify the United States as the country that first developed the atomic bomb, and only 57% of all Americans polled could correctly identify it as such).

As for Truman, it's actually a tricky sort of thing. After he initially heard that the bomb was dropped on Hiroshima, he was elated. He continued to be elated until he received reports about the casualties, which were much higher than he apparently expected. After the war had ended, he stuck to a position that ultimately he had saved lives (a number of lives that he continually increased over time, without apparent reference to any particular official analysis), and said he never regretted it.

The pilot of the Enola Gay (Paul Tibbets) was always a true-believer and always said it was the right thing to do.