He is a captain-class Shinigami. He isn't weak.
He does, however, have room for improvement, as he is significantly younger and less experienced than his Captain-peers. Kyoraku has stated "Hitsugaya is a genius. Given a hundred years (which is not a long time to Shinigami), he probably will be stronger than I am" which is something, as Kyoraku is one of the older Shinigami.