I wouldn't say he was "terrible" -- I'd just say he lacked ability. But that's exactly the fitting to Nirvana's music. If he went off into Van Halen like solos, then it'd affect the music -- it wouldn't fit and it'd be less than stellar, to say the least. Technically, no, he wasn't "good." Artistically? One of the most innovative of the 90's. Listen to songs like Radio Friendly Unit Shifter and Milk It, and you'll see what I mean.