A mirror very seldom simply reflects reality: it distorts it, flattens it and even reverses it. Not to mention the fact that when we look in the mirror, we are as human and fallible as ever. Even if mirrors were perfect representations of what we looked like, we still see that reflection with a biased perspective.
I admire people who remove all mirrors from their home, they must lead a far more stress- free life. I find that, for many people, checking mirrors is often an anxious experience that creates extra stress rather than pleasure. Even when I see people who look both immaculate and beautiful, they still seem to see something to frustrate them in the mirror.
How many of us don't? How many people look in a mirror and say yes: argh yes, perfection- exactly what I'd hoped for. Whether it's as small as a hair out of place, or a feeling of body dysphoria, most of us dislike something about our reflection.
Mirrors also don't really show you the truth though. The angle, the lighting, the reversal of your reflection, the reduction to 2D, the size... all effect how your reflection is recreated in front of you. They're useful sure, they can help you 'fix your face', or smarten yourself, or whatever you want to do, but they're not infallible. They're an inanimate object, not a messenger to dictate your looks to you.
Even more importantly, when you look in the mirror, you are also looking at your harshest critique. We all recite this as fact but are pretty terrible at actually applying it to yourselves. No one will notice if your skin is not 100% smooth, and guess what? Almost no one's is! We notice flaws in our appearance that no one else will ever pick up on.
You may walk away from the mirror feeling terrible, but the mirror lies. Most of us see ourselves in the mirror at out worst, see our flaws and 'imperfections'. What you don't see is how you look when your face light up in laughter, or when you push your hair back from your face, or the bounce of your walk. In short, you don't see yourself as others do.
Beauty is very seldom perceived in a mirror; it is perceived in motion, in 'real life' and so often by others. Finding yourself beautiful is no crime though, it is not vain to love the body that does so much for you. Keep the mirror if you want to, but be wary of it's lies.