Can a ballad be historical or nonhistorical?

Yes, a Ballad can be either one, or a combination of both. Some ballads have been based entirely on a historical event, which was then put into ballad form so that the event it described would be more easy to remember: in the centuries before there was widespread literacy, songs (and poems) were a useful way to transmit historical stories.

It should be noted, however, that even when a ballad is based on an actual event, the telling of the story may not be accurate (as with TV documentaries and historically-themed movies, sometimes the author takes some dramatic license to make the plot more exciting). There are ballads sung about many famous historical heroes and villains, from Robin Hood to Jesse James, and some ballads about historical events have become hit songs, such as the 1976 ballad by Gordon Lightfoot, "The Wreck of the Edmund Fitzgerald." (As mentioned, while the song was about an actual event, some of the details in it were not entirely accurate.)

There are also ballads that have nothing whatsoever to do with history. They tell a story about love, or romance, or patriotism, or any of a number of topics that inspire songs and poems.