Actually from what I saw, Rey wasn't wielding the lightsaber in all that practiced or disciplined a way, and seemed to be relying more on anger or the Force than anything else.
In fact the impression I get was that Kylo hadn't really faced down anyone with the Force before, and was pretty much caught off guard and completely unprepared for how strong she was.
Whatever the case, I certainly didn't think it diminished his character or the threat that he could ultimately pose. And plus I really like the idea that Rey and Kylo will both be training harder in order to fight each other in the future, and that Kylo isn't already established as a major Vader-level villain from the get-go.