That was the developer's fault. Sure Antasma wasn't portrayed right, but he could have been handled a lot better. I don't buy that "no potential" thing. They could have made him a bat possessed by the spirits of the deceased villains in the series, considering he shares traits with them, most noticeably the Dark Star.
Bowser just doesn't seem like the type of villain to pull a Dimentio like that. I know you hate Dimentio, but at least he hinted at his betrayal throughout the game and was developed as his own villain rather than an ally of Bleck.
I do like Bowser's portrayal in Dream Team for the most part but would it have been so hard to make him the main villain and Antasma a side villain? That way Antasma probably wouldn't have died and could have possibly gotten the Fawful treatment later, and Bowser would have had more chance to develop the way his character was intended in the first place.
Why do I get the feeling you like the twist only because it was in Bowser's favor anyway?