Imo mirror selfies do on average tend to look a lot better. I think a lot of it must be that the photo is taken from further away. This causes two things...

1. The picture isn't a detailed because the shot is simply further away. Wrinkles, acne, and other imperfections are not as clear or pronounced.

2. Features like your nose, chin, eyes, etc. appear smaller in far shots than close shots. In close shots, there is a bit of a "fisheye" effect due to the perspective, even if you aren't using a fisheye lens. It exaggerates a lot of facial features and isn't how you normally see yourself when you're looking into the mirror because you just aren't that close.

No, it's just just "because the image is flipped" which is what is repeated ad nauseum online. The biggest thing is the second point I mentioned.

There was a gif out there somewhere that very simply and easily demonstrates this phenomenon of how wildly different your facial features can look from this.