Godzilla is a metaphor (either intended or simply ingrained in the Japanese psyche) for Hiroshima and Nagasaki
From what I’ve gathered, not quite. The film showed up around the time of the Castle Bravo tests at the Bikini Atoll. The bomb tested there turned out to be dirtier than predicted, and fallout made it to some Japanese fishing vessels. It became a bit of an international incident.
And then the original Gojira film launched. And one early scene showed a fishing boat, which went under in a bright flash of light.
Gojira, or Godzilla as he was westernised, was not just the personification of the bombs at Hiroshima and Nagasaki, it was the personification of the fact that this could happen again.