Yeah Nat Portman could/can act but she didn't much in TPM or in the PT in general (that's pretty much down to Lucas only Ian manages anything like a performance in that film and that's in a cast with seasoned actors with stage experience).
Leia is wearing those clothes for the same reason the other Rebels in the command centre are...it's freezing cold, what should she be wearing exactly?
She wore a nightie in ANH because she was captured while doing her Senatorial princess undercover work and it was warm on Yavin, She wore a bounty hunter disguise in ROTJ to get into the palace and a metal bikini because Jabba is a fat pervy slug. She spent the rest of the film in combat gear (the only weird dress choice in that one was the Ewok's little Earth goddess number...Ewoks eat the men, worship the droids and dress up the girls). The Bespin Dinner outfit was a strange choice for evening wear but at least she got out of the frozen weather outfit. On the whole Leia's clothing makes perfect sense.
A lot more sense than dressing Jedi in Moisture Farmer outfits which restrict movement and going to the trouble of putting on forest camo and then dragging along a gold reflective fussy protocol droid (even if he did turn out to useful).