folk music IS an art, therfore folk musicians ARE artists..
I presume you mean to ask about musicians who also work (play, excell)
in OTHER medium...

I remember that Rick Fielding did carving in leather,
and many of us are right-brained creative types whose
expression takes many forms...

word crafting(writing), fiber arts, silver smith--
and other jewelry, pottery, and many other medium
are among the ways that singers and others express
themselves.

Greg Brown has designed "album" covers of his music,
and Joni Mitchell also paints.

the list goes on, and the beat goes on...