I think that is a bit of an ad hoc fallace.

Sure, because Obama won we declare him the most charismatic candidate. Had he lost, people would say the oposite, declaring him 'professorial' 'aloof' or 'elitist' (all criticisms directed at Obama at times in the media) and point out his lackluster performance in the first debate as evidence of a lack of charisma.

Perception of charisma is also heavily influenced by our personal biases. I'm sure that we perceive candidates we like as being more charismatic. I bet a majority of Democrats would say Obama is charismatic and a majority of Republican would disagree.