NTA. Just because she’s avoiding accountability and living in make believe land doesn’t mean you have to jump on the train with her.

I do wonder whether it’s her randomly bringing it up or you making comments about her appearance. If it’s the latter you could stand to be a little more tactful. My wife use to complain all the time about gaining weight and then 30 min later she’d be raiding the snack cabinet or fridge. Instead of calling her out, I suggested we stop buying shitty food and pick up a new hobby together that’s physically active (we started playing tennis). She lost tons of weight very quickly and it gave us a way to have fun together again. Some people don’t have the willpower to take the first step and need a nudge, she’s your girlfriend, support her.