Warum ist das so?

This is mainly due to the Mere-Exposure effect, with https://www.swr.de/wissen/mere-exposure-effect-100.html

On photos we often find ourselves not as beautiful as in the mirror. We actually look the same. Right?

Why do we find our mirror image so good?

Usually we find our mirror image pretty good. This can be explained by the so-called Mere-Exposure effect: If we are confronted with a certain stimulus – for example our mirror image – more often, then we find this stimulus with time ever better. The more we see something, the better we like it and our mirror image we usually see several times a day.

Why don’t we like photos so much?

On photos we usually see each other feeling wrong. But actually, we’ll see each other as we actually look, so how others see each other. In the mirror, we’ll see each other in mirror. When we look at photos, it’s uninhabited, so we’ll find it less good for the first time.

So that’s normal.