Don't worry about it, Gypsy. When you are playing the dulcimer, your "right brain" (spatial skills) is going a mile a minute, and your face is on its own. That's why some musicians make faces when they play - the brain is busy doing something else, and the facial muscles go uncontrolled.

When the number's over and the applause starts, smile then.

I bet the people who criticize you either don't play an instrument or they never play anything challenging.