Whooowee, a tough one -- Jones didn't exactly hit with the crown of his helmet, but he did lower his head coming into Kamara and hit him with what looks to me his upper forehead.
The main force of the hit all came from this head contact -- Jones' head was the first thing to hit Kamara, it looked like the hit was intended to be delivered primarily with his head (Jones didn't exactly follow the hit with his body, his arm kinda glanced over the top of Kamara and Jones's momentum nearly carried him past Kamara).
So I think this is a legal hit, however it didn't really live by the spirit of what all football players are taught in terms of tackling technique -- head up, deliver the tackle with the use of the entire body, tracking the near the hip, maintaining leverage, or hitting the thighs of the ballcarrier with the leverage shoulder, wrapping up and driving, etc).
Here, Jones' helmet isn't a piece of safety equipment but used as a tackling accessory. While Jones's hit can be called legal, it's still problematic IMHO from a safety perspective given the intent to use his main, if not sole, means of bringing Kamara down. Delivered to the side of Kamara's own head, not surprising this would cause a concussion whatsoever.