Generally speaking, a communist government in China positively affected U.S. relations with japan, since this was during the Cold War, when communism as an ideology was the United States' main rival.
It affected their relationship positively. The United States understood that Japan was a much needed ally that could help in that part of the world and keep the power balance in check so China wouldn't try to completely dominate the region. This helped Japan develop rapidly following the destruction it suffered, while also keeping the US secure that nothing too bad might happen.
