Yes, political cartoons are a valuable historical source. However, we must be sure that we do not overdo our confidence in them as sources.
Political cartoons are valuable because they can tell us something about the attitudes that people had at a given time in history. For example, let us look at the political cartoon that is shown in the “harpweek” link below and discussed in the eNotes link. This comes from 1888. If we look at this cartoon, we can see that the issue of “trusts,” or monopolies, was a major issue for the people of this time and place. We can also see that there was some degree of distrust of these large companies on the part of the people. The cartoon shows us this because it shows Carnegie portraying a monster as a “trustworthy beast.” The cartoon is helpful, then, because it shows us that this was a major issue and it shows us how some people thought about that issue.
But we do have to be careful when using these sources. We have to realize that not all people would have agreed with the cartoon or even cared about the issue presented in the cartoon. Therefore, we cannot be too confident in using cartoons to tell us what people thought. They might or might not have agreed with the content of the cartoons.