As others have stated, Kylo demonstrated powerful abilities and was shot by the bowcaster when he lost to Rey. I also think that Kylo losing to Rey had more to do with Rey being a Mary Sue character than Kylo being weak. Rey could do anything perfectly. If Darth Vader and Emperor Palpatine had both been present, Rey would have whooped them both at the same time just because whoever wrote the TFA gave her a complete inability to do anything wrong. She's like Superman without Kryptonite, and I think that had much more to do with her victory than Kylo being weak.