Stetson Kennedy was able to go undercover and expose the nasty inner proceedings of the KKK. This organization is notoriously vile, but Kennedy helped to expose all of secrets of the Klan. This was one of the best attacks against the organization and helped to educate people about the organization. He exposed the ways that they communicated from passwords to obscure hand signs.
The KKK is an organization that has caused exceeding amounts of damage to black communities across America for generations. Any action taken against this organization is positive since all humans are created equal, regardless of ethnicity. Stetson Kennedy could be considered a national hero for his efforts, and could even be considered more than a national hero as the negative effects of the KKK have stretched into other countries such as Canada at times. Many will benefit from his actions.
I believe Stetson Kennedy is a national hero for infiltrating the KKK because he was able to expose many details about the KKK organization that were otherwise unknown to the public. He was able to open a lot of eyes to how violent and cruel the KKK was/is. Without Stetson Kennedy the public may not have known the extent of the damage the KKK was/is doing.
There's no doubt that Stetson Kennedy was a brave soul, and he did important things at a pivotal point in American history. But I don't think that he should be called a national hero. The politics of hate tend to have a short shelf life in America, and if he hadn't uncovered the Klan's secrets it would have been somebody else.