The speaker looks at a world that seems obsessed with difference. The things that set people apart are loved by some and condemned by others, and the speaker wonders why. Fear of the unknown, or fear of change, is offered as a possible answer, though only as a guess, with the question marks left hanging.
Then the poem turns on itself. The speaker first says we are all the same deep down, so why should anyone's differences matter? Right away, "Or are we?" pulls that idea back, since every person is also unique. The two ideas never settle into a winner. The final lines, "Everything at once / And yet, nothing at all," keep both ideas alive. The title fits: a two-sided mirror shows you yourself and the other person at the same time.
The mood is thoughtful and gently puzzled. The speaker isn't angry or preaching, just honestly stuck between two true-feeling answers.