I mean, its bad enough, and kinda silly to walk around in her undies all day with "the boys"
I get the distinct impression that the Major deliberately uses her (prosthetic) body as another weapon to distract, manipulate and disarm. Thus her standard mode of dress.
There's an episode in season 1 that I think illustrates her mindset, where early on the Major gets covered in garbage water, so she has to temporarily change into the most readily available clothing. That outfit turns out to be a much more 'seductive' outfit than her normal outfits, which though equally revealing, have a more blatantly in-your-face impersonal quality to them.
The Major is notably uncomfortable with the more 'seductive' outfit and the emotional connotation it suggests. She exposes her body normally in a purely impersonal manner, almost as a weapon, and is uncomfortable doing so in a manner that even suggests emotional involvement on her part.
I think there was also an episode in Season 2 where the major has to 'sleep' with a government minister in order to gain intelligence - except she doesn't actually sleep with him. The Major's totally comfortable stripping down for him and getting into 'bed', but she then knocks him out before they get intimate, and she expresses disgust at the idea of actually having physical contact with him.
Once again, she has no problem flashing her body to distract, but doesn't want to do anything that gets too close and personal.