This question was at the center of the conversation of a group of kindergarteners that I sat with yesterday while we drew and cut. (I am an assistant teacher in a K classroom in Brooklyn.) One girl commented that Mulan doesn't wear a dress. Another girl replied that a princess does not have to wear a dress.
Most Disney female characters are, in fact, princesses (or mermaids in very revealing outfits) so I could understand how the children would be thrown off by Mulan, who disguises as a boy so that she can go to battle in place of her father.
I love all the open-ended conversations that take place in Kindergarten. My role is to ask the children questions to elicit their thinking on these topics.
The next topic of conversation was "Are mermaids real?" Instead of answering the question, I asked each child at the table "What do you know about mermaids?"