In hopes that you really are a loser and will run straight into 'em
I assume that the "levels" aren't actually made by Bowser, but rather, he is simply making use of them, in an effort to protect the keys. **The same question can be asked of why Bowser fights in places designed like the ones in the original Super Mario Bros, where he gets dunked into lava, with a similar answer - he didn't make those levels, he just utilised them.
You'd think after the second time, he'd at least choose a different type of venue...
"A gentle sea dragon lives here. Pound on his back to make him lower his head. Don't become his lunch" (talking about Dorrie). What's this about becoming Dorrie's lunch?
Supposedly, you could slip off of Dorrie's head before he raised it, resulting in Mario landing in front of his mouth and getting eaten accidentally. As far as this troper knows, this wasn't actually possible, and was either just a joke, a rumor, or perhaps a small feature that got scrapped in the final game.
Are the painting worlds entirely created by Bowser, or are they links to places that already exist?
Considering the Throwback Galaxy, I'm inclined to believe it's the second option.
This troper thinks of the courses as the paintings themselves, and Mario as an intrepid fictioneer. It would explain how he is able to jump between different points in time within the painting world, apparently temporarily replacing the past version of him who originally obtained that particular power star, but how it will just be an outline and history will only be rewitten if he succeeds.