Political cartoons are a very good historical source. However, we should be careful not to read too much into them.
Political cartoons are a very good source for historians because they can teach us a great deal about what people thought about certain issues at certain times. For example, looking at the cartoon in the link below that has to do with Haiti, we can see a few things. We can see that people in the US at the time were prone to having racist attitudes. This is shown in the caricature of the black baby. It shows that Americans believed that the government of Haiti was very weak. Finally, it shows that Americans believed that it was acceptable for the US to intervene in the affairs of other countries.
All that said, we must be careful about attributing these views to all Americans. A political cartoon is just the view of one person. We can assume that other people felt the same way that the cartoonist did, but we cannot be sure.
Thus, political cartoons are a great historical source as long as we are careful not to assume that all Americans of their time would have held the same attitudes that the cartoonist did.