I'd bet Joe the Plumber had only a general knowledge of McCain and his history and personality, then he learned much more of what Juan is like by being with his campaign for a few days. Only a minority of the population is knowledgeable about DC and the characters in play there day-to-day.
Joe was suddenly thrown in with all the sharks and posers and schemers of presidential politics, and he hasn't done that badly all things considered.
Joe has no one to blame but himself for his alleged hellish' life.
I doubt that Joe was that uninformed but even if he was, it was his responsibility to bring himself up to speed.