I think it's best to look at some of the other photographs that show her in that uniform:
(The styling of the uniform is different, but keep in mind that it's hard to see that yellow middle part of the uniform in black and white photographs.)
Take a look at this part of the Wikipedia article:
"Darger's human figures were rendered largely by tracing, collage, or photo-enlargement from popular magazines and children's books. Some of his favorite figures were the Coppertone Girl and Little Orphan Annie."
I think that Darger found a photograph of Tatiana wearing that outfit. He then created a character in his story that was inspired by her. In that case, is it possible to determine exactly which Tatiana photograph was the inspiration?