Bowser is under-used and over-used at the same time imo.
Under-used because every time he is used, he is usualy pretty much a joke, with no real motivation or personality, making him hard to care for him.
Over-used because despite the fact that the Mushroom Kingdom has the potential of being rich with villains, somehow, it is ALWAYS him that we have to face in every Mario game. I wish we could face Donkey Kong, Wario and other villains once in a while. Link faces villains other than Ganon, even in main games, this is why facing him still feels like a big deal even after 25years.
So combine those 2 aspects and you get the lame ''Bowser kidnap Peach for no reasons again and I can't get myself to care'' storyline over and over and over and over again. Result: I'm sick and tired of seeing him in my games, I want LESS Bowser.
What?! I can't hear you over all this awsome! - Pyrrhon (Kid Icarus:Uprising)
Final Ultimate Legendary Earth Power Super Max Justice Future Miracle Dream Beautiful Galaxy Big Bang Little Bang Sunrise Starlight Infinite Fabulous Totally Final Wonderful Arrow...FIRE! - Wonder-Red (The Wonderful101)