Its an experimental SMILE fruit deemed a failure by Vegapunk. Lots of performance issues could occur and he doesn't seem to have a control over it because its artificial. Its almost acting on its own like a separate entity, and unstable.
His goal is to transform into a bigger dragon.
Let's be honest your formative years from child to adulthood are essential to become stronger especially in One Piece. It will be The Force awaken bad writing level if Momo suddenly becomes strong. Luffy literally trained like hell since he was a child to to become powerful. Garp made him go through insane shit to make him tough. He learned to survive a wild environment with Ace and Sabo and perfected his df. Zoro trained for almost a decade to become a proficient swordsman to take on the dangers of the Grand line.