This feature from Star Wars: The Force Awakens Visual Dictionary makes it evident that this is wrong - he was really quite powerful:
As the feature notes, he has "deadly lightsaber skills", killed all of Luke's students, "is the most gifted apprentice of Supreme Leader Snoke" and is "an almost unstoppable force on the field of battle".
(via Kylo Ren's Lightsaber Design Explained For Star Wars: The Force Awakens - Cosmic Book News)
