For my own situation I don't rate a mirror as especially important. One reason is that it needs sun. Right now I look out of the window at noon in summer and I have no sun. Another reason is that I live and work in an urban setting, so there will usually be someone within hearing range of a good whistle.

So a mirror is something I am willing to downgrade. My EDC doesn't include one at all. My small kits include a piece of mirror card cut to fit the kit. It's actually made of plastic, not card, about a 1/3rd the thickness of a proper signal mirror and a huge improvement over a polished tin lid. No aiming spot, though. If you are looking to save space it is something to consider. I'm not recommending it as such because I don't know how important a mirror is to your situation.