The Magic Mirror presents Farquaad with 3 princesses who he could marry to be King. One of them is Cinderella. But Cinderella wasn't born a princess. So what gives? Did DreamWorks make her a princess by birth, like Disney did with Rapunzel? And if so, would Cinderella's stepmother be an evil queen like Snow White's was?
The problem with Cinderella as a character is there's no one single canon story to point to (unlike Snow White). As Wikipedia notes, some versions have her of noble birth
The twelfth-century AD lai of Le Fresne ("The Ash-Tree Girl"), retold by Marie de France, is a variant of the "Cinderella" story in which a wealthy noblewoman abandons her infant daughter at the base of an ash tree outside a nunnery with a ring and brocade as tokens of her identity, because she is one of twin sisters the mother fears that she will be accused of infidelity (according to popular belief, twins were evidence of two different fathers). The infant is discovered by the porter, who names her Fresne, meaning "Ash Tree", and she is raised by the nuns. After she has attained maturity, a young nobleman sees her and becomes her lover. The nobleman, however, is forced to marry a woman of noble birth. Fresne accepts that she will never marry her beloved, but waits in the wedding chamber as a handmaiden. She covers the bed with her own brocade, but, unbeknownst to her, her beloved's bride is actually her twin sister, and her mother recognizes the brocade as the same one she had given to the daughter she had abandoned so many years before. Fresne's true parentage is revealed and, as a result of her noble birth, she is allowed to marry her beloved, while her twin sister is married to a different nobleman.
So it's not unreasonable to make her a princess in this case, for the sake of the story (that doesn't involve her).