It's really nice; I like the way you've shaded the bottom of the dress especially, gives it sort of a 3D look. However, the folds are a bit strange on the top part of the skirt; they seem to end very suddenly and the curve is really sharp. Try to make it look more smooth.
In general, it's a good doll, but I'd suggest adding hair and a face, because with just a dress it looks a bit bland and incomplete. You could still have it be a mannequin; some mannequins do have wigs and faces (like these: File:Mannequin Supermodel 2.jpg - Wikipedia, the free encyclopedia
). Maybe recolour the skin so it looks more obviously fake and plastic-y.