This is actually Marathi (a language of Maharashtra, India) idiom.
It has very deep meaning. Why we use mirror? To see our face which we cannot see without it. A mirror (normal) is a reflection of true image of ours. We cannot hide any thing from mirror, even if try to, with respect to how we look and what are we actually.
We do not need mirror for those things which we can easily see. So, if I can see some thing without mirror than why I need it? So if I know that my attitude is wrong, why I need someone to act as a mirror and try to understand my reflections. It can be seen any time by me and others as well.
This idiom is used when someone is trying to hide or someone is trying to pretend that he / she needs a mirror to `see/observe` the fact, when the fact is wide open in front.
Have a great life..