This costume is on screen for maybe 30 seconds but it is during one of the most important scenes in the prequel trilogy: Chancellor Palpatine rebrands the Republic as the Galactic Empire. Padmé watches in horror.
As far as shape and fit, this gown, like all of her Senatorial looks in Revenge of the Sith, is cut to conceal her pregnancy with layers of bulk in fine silks, brocades, and velvets. And we barely see anything above her shoulders anyway.
Nevertheless, this costume has quite a lot to say.
First of all, the color choice is significant. Purple is often associated with royalty or aristocracy and symbolizes luxury, nobility, power, ambition. These connections fit the scene and the entwined character arcs of Amidala and Palpatine. But perhaps more importantly purple is a combination of red and blue. In the original Star Wars film blue and red were shorthand for good/Jedi and evil/Sith. In this film the struggle between them is embodied in Anakin. Thus, the purple of Padmé’s gown, worn during the explicit downfall of the Republic, ties her to Palpatine, Anakin, and also Mace Windu, who wielded a purple lightsaber and whose death began the Jedi purge.
This outfit also appears in the Clone Wars series, here on Naboo, and later in a Senate scene reminiscent of the film one. Above we see her again beside Bail Organa, too, and though it is not the same of his looks, he is wearing blue as he is in the film.
But the most notable part of the costume is the hairpiece.
As seen on screen, from the front and side, it most resembles the upsweep of a crescent moon, something associated with transformation and woman/motherhood (also love and justice if you’re a 90s kid). Or it can be read as wings, such as an angel from the Moons of Iego might have, or the symbol of the Jedi Order. In fact, it directly references the Rebellion:
Several scenes depicting a group of Senators lead by Amidala and Organa allied against the Chancellor were removed from the final cut of Revenge of the Sith. This lost subplot confirmed that Padmé was instrumental in the founding of what would eventually be known as the Rebel Alliance against the Empire.
This costume is on screen for maybe 30 seconds but it makes a statement that echoes through generations.