They don't actually see nothing wrong when they look in the mirror. They know that they are ugly, hence the overcompensation and caking on makeup, adding contour lines, etc. The whole "body positivity" thing is an exercise in self delusion in the manner of existentialism or post modernism: they think that if they can convince the world they are attractive, it will actually make them feel (and be) attractive. This inverted world view also implies that they only feel bad about their appearance because others judge them on it, which is again obviously false to anyone who isn't deluding themselves.