My son does that.
He’ll pick apart his appearance mostly - he’ll criticize the tiniest feature, things other people wouldn’t notice. His current things are a couple very small scars on his face and that he thinks his hair is thinning.
I think part of it is that they think everyone else is looking at them and judging them, so they want to see what others are seeing.
Part of it could be that their perceptions are off, so maybe they look strange to themselves.
My son’s taken to looking at pictures when he was very young over the past year.
He’ll sometimes say stuff like “I was a cute kid” as if he can’t believe it now.