Not to go all "sharp knees" but... okay yep I'm going to do it anyway.
That dress is exactly what someone like her should wear, because the additional fabric to the sides (as well as the curve of the green line) is meant to give the appearance of hips, and turn a boyish silhouette into an illusion of feminine shape.
Left: with the fabric flows around her
Right: approximate reality
The additional fabric illusion is evident as she turns, and you have a nice view of the added piece here... along with her totally flat backside:
With enough make-up and strategically cut clothing, she looks okay on a red carpet for a minute... but in pretty much every shot of Dune, she's about as androgynous and unappealing as possible.