The de Sèves use a princess (although they are referred to as Duke and Duchess, their fathers are kings, so they are really a prince and princess) to sugarcoat a fairly standard, if worthwhile, message for children: get to know people before you judge them.
The vibrant Duchess has a court of quirky characters who party non-stop. Nothing is too wild, crazy, or original for them, they can never have too much fun and excitement. Poor Norm has a mad pash for the Duchess, but she thinks he's a bore and only tolerates him because her father insists (for reasons of international diplomacy). But when her cook falls ill, she discovers that Norm can grill a mean cheese sandwich and decides that maybe ordinary isn't always bad.
I kind of wish the story had continued a few more pages. Does Norm change Whimsy's party-girl ways? The narration is pretty neutral, but I was kind of disgusted by her extravagant lifestyle. It reminded me of the Pre-Revolution French Court, immense waste of money and food on a few spoiled individuals. And at the taxpayers expense! When Norm was introduced as caring about road maintenance and rescuing abused dogs, I thought there might be a political dimension to the story, but I was disappointed.
Also, and I realize this isn't the point of the book, I was bothered by the treatment/use of animals in Whimsy's palace. They wear silly costumes, they get manhandled, the sea creatures are in horribly tiny containers.
Probably neither of these issues would occur to children, but I'm not sure that makes it better. Neither "partying and spending money is how you have fun and get friends" nor "animals are like toys" are attitudes I'd really want kids to internalize, even subliminally.