Lmaoa, I remember reading about this.
If I recall correctly, a picture shows how you appear to others whereas the mirror shows you a 'mirrored' image of yourself (e.g., your left side is the reflection's right side).
We are used to seeing our reflection, which then makes pictures of ourselves look strange.
I think selfie cameras intentionally take a mirrored image to avoid this effect. But I've never taken one because I hate looking at myself.
There's a similar phenomenon with hearing ourselves in recordings vs. as we speak.
Edit: as @ch00f@lemmy.world already explained!
Are you sure it said asymmetrical?
I read something on the topic (though, awhile ago now) and it talked about symmetry being attractive, and even gave Denzel Washington as an example because he had a remarkably symmetrical face.
Wud be really interesting if asymmetry is found attractive now!