Yes, the United States should have joined the League of Nations.
The League of Nations was created after World War I and was made by the countries that had won the war. They wanted to try to keep the peace and hopefully prevent another world war.
The catch is, the League of Nations was essentially useless. It was very weak and obviously did not prevent World War II from happening. The American public was not interested in joining, the League had no real power to enact any rules, they had no troops whatsoever, and the League was not representative enough to do anything.
While the League of Nations was very weak and useless, it was still a good idea. I believe the United States should have joined the League of Nations because of this. Since it was a good idea, the United Nations was made after the League ended. The United Nations is much more powerful and learned from the flaws that the League of Nations endured.