Personally, I think it's quite the opposite of cynical. Put in the proper perspective, "life's not fair" can be quite a liberating thought.
Trying to make life be fair can lead to a lot of frustration and "right-fighting". It can lead to a frustrated desire for punishment of wrong-doers and vengeance on people you perceive as "evil". And, at it's worst, expecting fairness from life, and not receiving it, can make you feel very ill-used and embittered. People sometimes tie their personal happiness to the belief that life should be fair, and if it isn't, something's wrong and needs to be fixed.
But when you accept that not every virtuous act will be rewarded, not every misdeed will be punished, and not every wrong-doer will suffer (or at least, not in the amount or way you'd like to see them suffer), then it's much easier to be calm and compassionate, forgiving and joyful.
Life isn't fair, and that's okay. Sometimes bad things happen to good people, and sometimes good things happen to bad people. It's noble to want to try and fix the wrongs you see in the world, but since life isn't fair, you're free to choose your battles and focus on the things that matter most to you.