To be fair, Donna dressing that way is apparently a plot point.
Gar says later to her: "And when did you start raiding Kory's closet?" To which she replies: "Nice of someone to even notice."
Best guess is that Donna's going through a midlife crisis due to the death of her husband and son. (Even though that happened a few years ago.) The crisis thing and the way Donna's acting all seems a bit out of character, but at least it's being done on purpose, which implies that the author knows that Donna would usually not dress or act like this.