It looks like Mon Mothma is wearing a simplified tabard variant of her Revenge of the Sith robes over a jumpsuit or leggings (the side fastenings of the tabard may be concealing a waistline, but Rebels is very pro-jumpsuit.)
Ceremonial but practical, this is a neat way of maintaining Mothma’s air of dignified authority whilst logically placing her mid-action. I believe that by this point Mon Mothma will have left/fled the Imperial Senate, so this look is very much a statement of intent that she is continuing her role as senator and representative of Chandrila, even as she is fully and openly wearing the role of Alliance leader. A militant adoption (arguably along similar lines to the influences of Padme’s Mustafar costume.) This is also another case of adaptation of costume from one medium to another: live action to animation. And within that, the fairly rigid aesthetic of Rebels which is made of clean simple silhouettes, leaning towards the utilitarian.