Nay.
Mirrors can cause aggression, unhealthy fixations, abnormal behavior, socially bad behavior....
If you have one bird for 5 or 10 years, keep it by itself, and it always has a toy mirror, then one day you decide to get another bird, that second bird will not look, nor act, like the mirror buddy will. This may cause the first bird to attack the second bird, but the second bird isn't "right".
I met one cockatiel that had such an unhealthy fixation to his mirror, that he only moved to eat and drink... he was fed a horrible diet, in a tiny cage that was probably never cleaned and he never received any baths. He felt like he was living next to a stove for 10+ years.... you know that grime that builds up around stoves if you don't keep them clean? A thick, greasy grime? Ya... that's what he felt like...
And mirrors in a flock situation? Not necessarily any better! It can cause birds to pay more attention to their mirror buddy than to their flock mates!
I've told several people of the possible dangers to mirrors. I've had several people disregard my advice and provide their birds with mirrors... and then those people had issues and had to remove the mirrors. These people then came back to me and said "Monica, you were right". I don't ever want to tell someone "I told you so"....
I do not put mirrors in cages. I don't keep the birds away from mirrors 100%, but I don't make it a point to have them in front of a mirror, either... The only places that have mirrors are the bathrooms, and if I have a bird with me in the bathroom, I might show them the mirror, point it out, tap on it, see what their reaction is, and they generally could care less! Most of the time, I don't even acknowledge the mirror if I have a bird with me.
I don't feel as if mirrors are good for birds, not mentally, nor physically. Not with what I've seen and read from others.