if the shadow king hadn't been evil, it's greatly possible that the x-men wouldn't have been formed. for him to have been good, and later a part of the x-men, they would've had to meet under different circumstances, and xavier would have had to come across another uber-powerful 'evil mutant.'
the shadow king has many times been a part of x-men history. whether that makes him interesting or not is based on the opinion of whoever's reading the books. historically, the shadow king has not been a deterent in selling comics. if anything, he's drawn in readers who stopped reading the comics sometime in the past, and who were attracted to the familiar face.
popularity ratings on that site are based on character relations, not on public opinion, if i recall. he's got a negative number because he's 'evil' and an opponent of the good guys, not because he doesn't have a fan following.