Should the U.S. have done more to stop China from becoming communist in 1949? Why/why not?
No, the United States should not have done more to try to stop China from becoming communist. Outside of any ethical issues about trying to determine another country's future, the US had no chance of actually preventing the communists from taking control of China.
To think about this, just consider the Vietnam War. In a country a fraction of the size of China, the US was unable to suppress a communist insurgency that had the support of many of the people. If the US had tried to get involved in China, the same thing would have happened. Mao and the communists had enough support that it would have been very difficult to defeat them. American intervention would not have helped make Chiang and the Kuomintang any more popular as it would have seemed like a return to Western domination.
Any attempt to intervene militarily in China would have been unsuccessful so there would have been little point in trying.