First off, the folks costuming Genevieve O'Reilly need all the Emmy's...
Obviously, with the costuming of Mon Mothma, she's shown to be Chandrilan old money, and pretty well off. It's inspired by Eastern fashion, namely Japanese stylings, which is fitting for some of Lucas' original inspirations for Star Wars.
Someone else noted that her hair is getting shorter as the series progresses, and we know in RotJ, she's in the pixie cut.
Something else I've noted, and it maybe a nothing-burger, but....
Her clothing gets more and more austere as time moves on as we've seen from Clone Wars -> Andor -> Rebels -> Clone Wars -> RotJ
Part of that is likely simply living within her means and being cut off from her fortune. I'd like to submit that it's also a cultural influence on her as well.
By Rogue One/RotJ, it seems pretty clear she doesn't have her husband or daughter around. In the wedding, Leida's braid is cut off as a cultural significant act of Chandrilan marriage.
I submit that why we see her in the garb she's in in RotJ/Rogue One and her short hair, is culturally significant to her. Many cultures see women cutting their hair and having a simple style as a sign of mourning. Further, white is the color of mourning in many Asian cultures, again in Japan, where much of Chandrilan style takes notes from. By Rogue One, she's lost everything--except the Rebellion and the Alliance. That sacrifice is a big reason why so many in the Alliance will ultimately respect and follow her.