Even though it makes sense for Ozma, as the supreme ruler of Oz, to be considered a queen (if not an empress), and Land confirms that "Ozma made the loveliest Queen the Emerald City had ever known," L. Frank Baum typically refers to her as "Princess Ozma." Of course, going by the technical definition, every queen is a princess (although not every princess is a queen). Still, the cultural expectation, especially in the fairy tale world, is for queens to be fully grown and princesses young. So calling her "Princess Ozma" is probably a way for Baum to emphasize her perpetual youth. Besides, royal titles in Oz can be rather fluid. Nick Chopper is an emperor essentially just because he wants to be, even though his fellow quadrant rulers are kings and queens. And there are small communities throughout Oz with their own kings and queens, as well as leaders with such unique titles as Lord High Chigglewitz, High Coco-Lorum, and Czarover. So, really, I don't think anyone is going to object to Ozma calling herself whatever she wants. Although I can't recall for sure, I believe Ruth Plumly Thompson switches between calling Ozma "Princess" and "Queen," while I think John R. Neill just calls her a queen.
Actually, Phil Lewin's Witch Queen gives an explanation for why Ozma is only a princess, in that Lurline's older sister Enilrul still technically holds the title of Queen of Oz. It's tempting to try to connect Ozma's proper switch from Princess to Queen to her father's official abdication in Lost King, but since no one (well, except Mombi and Lurline) knows that Pastoria is even still alive prior to the events of that story, I wouldn't think that would make any difference. So I think the most likely explanation is that Ozma is a queen (and a High Queen, at that) who prefers to be called "Princess" (by people who aren't close enough to her to be on a first-name basis, anyway).