Generally speaking, light enters the room from the south. If the mirror faces south, the person facing the mirror will be backlit, and the image in the mirror can't be seen clearly, so the purpose of looking in the mirror can't be achieved. Of course, it doesn't matter under artificial lighting environment (such as bathroom, dressing table, etc.). ).
In geomantic omen, what is the purpose of setting the mirror? The specific advantages and disadvantages should be actually considered. Didn't say it couldn't face south.