Can you put a mirror on the wall behind the door?

No, from the perspective of geomantic omen, the door is a lucky place, and it is easy to show wealth by putting a mirror; If the porch is the living room, the place diagonally opposite the door is the place where wealth gathers, so people who pay attention to Feng Shui are willing to put a mythical animal (a beast that only eats but doesn't pull) in that position.

The corner opposite the door is a place where evil spirits gather, and containers containing water such as fish tanks can be placed to resolve them.

Scientifically speaking, home is a private place, and it is easy to give people a psychological hint of "someone is in the room" as soon as they enter the door, thus destroying the harmonious atmosphere of the family.

Similarly, the mirror in the bedroom should not be enlarged, especially on the opposite side of the bed. It's easy to be scared when you are sleepy.

I hope this answer is helpful to you.