Yes, folklore is definitely on topic here. Please feel free to ask these questions at your convenience.
Whether a tag should exist is a potentially more tricky question. Conceptually speaking, tags should exist if they are useful. In this case, since most of our content concerns native-american mythology, a search for american or america would be inundated with those results instead of modern American folklore - good example where tags would be helpful. On the other hand, other countries from the Americas (the continents) might well have a cause to object if the whole tag is appropriated for only the United States.
Thus, I suggest naming the tag north-american-folklore, by way of analogy from european-folklore.
In any case, you will be able to create new tags at 150 reputation.