I disagree. He's always had his good days. As Yamen says, his mistakes are costly most of times though.
IMO, that's got more to do with Barca's system of play especially in the Liga. The way the attack and get the possession, it's very hard for their opponents to create chances let alone scoring goals, and the Zamora trophy is given to the keeper who has the lowest goals to games ratio, not necessarily to the best keeper.