A minor, very minor rocker with a rather interesting history. He was first introduced in the late 70s, if memory serves, by Columbia Records, as a synthetic pop idol named "Johnny Cougar". That campaign failed, so Columbia renamed him "John Cougar". That failed too and Columbia (again from memory) gave up. But the kid was tenacious, even if he wasn't very talented (I recall all the scathing reviews at the time) and remade himself as a sort of poor man's Bruce Springfield (songs for and about the working men) under his given name.
Politically, Mellencamp is your typical record industry idiot, but he is hardly a 'very minor rocker'. He has sold over 20 million albums, which include a couple of number one hits and numerous top 40 singles and a dozen platinum albums. He did get off to an embarrassing start with the record company trying to mold he into a pop star, but he has shed that image and became a critically acclaimed artist.