My experience, in the corporate world and in education, has been that when employees (or students) refer to their boss, teacher or professor as being mean usually the problem doesn't lie with the target of their complaining and if you were to be objective their "mean" most often can be translated in the real world to mean effective, demanding, efficient uncompromising, competent, incompetent or any other adjective of successful people.
As soon as "mean" is used as the descriptive bells go off in my head because having taught secondary education I know that "mean" is the favorite description adolescents use to describe anyone who has expectations of them.
For example adults will say my boss (or professor) is incompetent, demanding, overbearing, a perfectionist, unfeeling, unsympathetic, self absorbed, an as##o##...any number of adjectives before resorting to the "mean" card.
Really, "mean" could mean anything (excuse the pun), different things to different people and is subject to infinite interpretations and is a descriptive that I doubt mature people would use in the context of "getting ahead".
Now I don't mean to say that anyone will never encounter a mean person on their way up the ladder to success but it really isn't a grounds for criticism of anyone who successfully executes their job and is effective. If they truly are mean and you have looked into yourself to be sure that the perceived meanness isn't just your intolerance of what is expected of you or maybe an unfair assessment of their behavior then so be it - it will catch up with them.
In the mean time (can't get away from that word) the best thing for your personal and professional development is to throw words like "mean" and "nice" out of your career vocabulary and concentrate on being the best you can - if you accomplish that you may find some day that some people think your success was due to a mean streak! :-)