I don’t think he was really lackluster in the debates. He just came off as if he didn’t think he needed to answer questions. I’d say he was kind of taciturn.
Those weren’t real “debates”. The news media didn’t ask him any questions. In both “debates” he was given the least amount of air time of any of the candidates, and the questions they did ask were silly. Such as his beliefs in God, etc. That, coupled with his natural good manners, prevented him from jumping in and talking over the other candidates. That hurt him big time, particularly when he had to share the podium with a blowhard like The Donald.