I get it, you're already asking yourself: "How on earth is this a shopkeep?"
First off, we're not on Earth anymore, so that statement is kind of null and void. Also, you would be wrong if you didn't consider these hungry star-people to be merchants in at least one aspect.
You trade Starbits for additional levels.
You sell a valued item to receive a product or service, and this is exactly what we get when we interact with the Hungry Luma. Mario trades in all these Starbits, which act as a form of currency, by feeding the Luma till he's near exploding. Once he is full, he creates a new level or area, which acts as the service.
See my point? Sure, I don't rate him very high because he is a one-trick pony and the variety of services are next to nil, but I still count him (and anyone like him) in the category.
Image source: Funny Pictures