Not necessarily bad artists:

• Those who once stepped outside their genre I don't like into a genre I do like. (K.D. Lang - Constant Craving)
• Those who covered a good song by someone else. (Eric Clapton - Cocaine by J.J. Cale)
• Those who were written a song for by an artist I like. (Tina Turner - Private Dancer by Mark Knopfler)

Surely there are some that accidentally made one good song, or used up all their creativity making it, or it was about a unique life experience that channeled itself into the song. (maybe Gerry Rafferty - Baker Street)