* Most Disney princesses are fictional characters. They exist in stories and are not real people. So, in that sense, they are not "people in costumes" because they never existed in the first place.
* However, the actresses who play them are real people. They wear costumes and makeup to transform into the princesses, but their roles are purely for entertainment.
* Some princesses are inspired by real-life figures. For example, Pocahontas is based on a real Native American woman. But even in these cases, the Disney version is a fictionalized and romanticized interpretation.
So, while Disney princesses aren't "people in costumes" in the literal sense, they are real people portraying fictional characters using costumes and makeup.