Disney skews towards Northern Europe with most of its fairytale adaptations. I think surnames should follow the vibe of the movies as much as historical origins, otherwise it's less Disney and more straightup folklore. Some suggestions:
Belle is Belle Barbot. Barbot de Villenueve is the last name of one of the first published writers of the story though its origins are much older. Barbot is easy to pronounce and it's alliterative
Ariel probably has no last name as a mermaid (but maybe Ariel Neptune?) As a married human, Ariel Lavard. Lavard is a last name associated with an early Danish prince.
Aurora is Aurora Fountainebleau. Sleeping Beauty is all over the place. The names of other royal characters in the movie suggest a Hapsburg-esque lineage, so anywhere between Portugal and Austria. Fountainebleau means 'blue fountain' and it sounds fancy.
Cinderella is Ella Tremaine. Her stepmother Lady Tremaine is the widow of Lord Tremaine, Ella's biological father.
Rapunzel is a hard one. In the older tales she was the daughter of peasants, so naming her after a common raddish-like plant would be fine. In the Disney adaptation she's kidnapped (probably Austrian/German) royalty and would not have been christened after a raddish. I'm guessing Rapunzel is the name Mother Gothel gave her to hide her, so...her name is either Rapunzel Gothel or a true royal name like Adelaide Hanover or Frederica von Corona (named after her father, and her the kingdom as their last name).
Snow White is Snow Leblanc if her mother was avant-garde and actually named her Snow, or her name is something like Neve Leblanc and Snow is a nickname.
Jasmine is Jasmine Bobolonius. As goofy as it sounds Disney canon says the Sultan’s grandfather’s (the founder of Agrabah) last name is Bobolonius. So there we go.
Merida is Merida DunBroch - That’s a no-brainer.
Tiana is Tiana Chase. Leyah Chase was an old New Orleans restaurateur who was an inspiration for the character.
Elsa and Anna are Elsa and Anna Agnarrsdotter
Mulan is Mulan Hua or Hua Mulan if you want to be more accurate. It’s in the movie, though it’s pronounced ‘Fa’ there.
Voila!
