Tiana is my favorite princess, and this is one of my favorite portraits of yours.
Gotta admit to not necessarily liking the 1920's design of the "standalone 'princess' dress," but like you said... historical accuracy, drag it kicking and screaming back into the 20's, and all that Jazz. xD
Although, I love this portrait for a wide variety of other reasons. Love the pure joy on Tiana's face as she dances. Love the design of the dress itself (even if it's not my favorite style). Love the attention to detail (especially the lilypad; thought that was a nice touch). Love the fireflies. And OMG LOVE the colors!! Sweet Heaven Above, I absolutely love the rich, deep, colors and textures of Tiana's hair, skin and dress. LOVE the contrast between her dark chocolate skin and the deep, rich green of the lilypad dress. (In fact, most marketing and fan portraits of Tiana seem to give her lighter skin. ) Everything about it is just wonderful.