Natalie wore it better, too!
Many people don't realize that most of the heavily costumed Queen scenes in Episode 1 and 2 are played by Keira Knightly (who I believe is uncredited).
For most of Episode 1, and the first part of Episode 2, Portman is pretending to be an attendant to the Queen, or, in Episode 2, pretending to be a fighter pilot protecting the Queen's spaceship.