Set up a mirror and watch yourself while you play. Set up an iPhone or video camera and make videos of your rehearsals and performances, and then watch them carefully. This should cure you of the habit, because you will be aware of what your audience sees when you perform.
I have known orchestra players who have embarassing facial tics and expressions and care nothing about them. I suppose they can get away with it because they are in a large orchestra and often in an orchestra pit, and the audience does not see them or take notice of them individually.
However, I think that in the current era when people in the audience of every performance shoot their own camera phone video and put it on YouTube, musicians ought to be much more aware of what the audience sees of them. Exaggerated facial tics and expressions simply distract the listeners and detract from the music.
Perhaps you should also read about and learn to practice some of the principles of the Alexander technique, which musicians use to change their entire posture and the way that they carry their body when they perform, in the name of removing stress from the body, freeing the body up to perform at its best, and preventing muscle and joint pain from bad posture. You don't have to study the technique formally or in great detail. Once you become aware of how you use your whole body, regardless of what instrument you play, you realize that any unneeded physical stress you put on any part of your body affects your performance. Learning to put all that in balance will probably make you a more effective musician.