tarinelove- With a lot of Disney films, they're actually drawing from the time during which the films were made, not the setting of the film.
For instance, Cinderella has heavy 1950's influences, as evidenced by her hairstyle, makeup, and pink dress. Briar Rose/Aurora is the same way, though she's moving a bit toward the 60's. Ariel, you can tell, has heavy 80's influence with the huge bangs and enormous sleeves.
I always like to take these sorts of things into consideration when designing my costumes!