>>56160370But it was still good writing. It has been established numerous times ever since the original season that Pokemon can have drastic changes in personality after evolution. This isn't just an anime thing either; Pokedex entries reflect this as well.
It makes perfect sense that a cute little lizard which eventually becomes a "dragon" would have a rebellious teen phase and hot-headed adulthood. He eventually came back around which is a perfectly natural growth arc and a somewhat common trope in various media.