Paul Bearer was handcuffed by the gimmick, Percy Pringle was a great manager in World Class.
Actually, most managers are made by their gimmick. Downtown Bruno was great as a heel in Memphis, but Harvey Whippleman was a terrible manager in the WWF. It didn't help that most of his charges were jobbers, but his gimmick left no chance of him ever getting over.
I'd actually vote for Paul Ellering as the worst manager. The Roadies certainly didn't need a mouthpiece, and they certainly didn't need someone to interfere on their behalf. There was literally no reason for Ellering to be there.