SilverWook said:
It wasn't quite a ruse. She probably dressed and spoke that way as member of the senate all the time.
True, but even then the posh act and dress are disguising the fact that she's much more involved the Rebel military. The only time she uses that accent is when trying to fool Vader and Tarkin. We never see her addressing the senate, but we do see her directly involved with delivering stolen plans, viewing battle schematics, briefing fighter pilots on the details of a mission, helping lead an evacuation, a rescue mission and an attack group against an Imperial base and prove that she's handy with a gun.
Seems she's much more involved with the military side of things than the politics and she's certainly depicted as a fighter more so than a princess.
So, back to the point, the new rebel troop-esque costume seems appropriate, where as her in a royal gown looking like a Queen would not.
She looked really hot in the medal ceremony.
FTFY ;)