Question

Why is the mirror like god in the poem Mirror?

Answer 1

Maybe because it does not favours anyone it treats all alike.
You can surely add somethin' else..

Answer 2

Mirror is like a god and a lake because a mirror shows your real image and if some bad thing is there on your body your can clean or remove it off as god shows you your sins and corrects and forgives them and as a lake shows you your real shadow.