Second of all, he's a pretty bad singer, and I say that based on hundreds of live videos I've watched, plus the concert I went to. Dexter almost never sounds really "on" when singing live.
Third of all, Dexter's voice sounds good on record when he's yell-singing, but he sounds awful when he tries to sing low. He gets very nasally, doesn't sing from his diaphragm at ALL, and it's painful. I can't listen to songs like Kristy and Fix You, and that's a big reason for it. He has to yell-sing to make up for the fact that he's not a very good singer. PS: I've heard every Offspring song made.
If we're ONLY comparing him to other untrained singers who choose to smoke a lot despite their singing and choose to sing out of their range, then I guess he's okay. But that's a very small portion of singers that we're judging him against. In the grand scheme, he's a pretty bad singer.