Simply dreadful. Clover decided that a dark and gritty Bowser is the way to go for this movie, and as such, Bowser comes off as just a heartless bastard that nobody could like. This would be fine if this wasn't intentional.
Bowser has always been a joke villian in the Mario series, where he just basically plays with Mario all the time. This retcon is such a bad way to make the movie more serious and gritty. Why the hell does it need to be like that? Mario is light-hearted and fun. Any attempt at dark and gritty should be a parody.
By making this too serious, it comes across a joke.