I once knew a guitarist who looked stupid when he was concentrating. His mouth fell open and sometimes his tongue lolled forward, his eyes stared into space..... No-one liked to tell him! He was a thoroughly nice guy and intelligent, and a very good lead guitarist, and no-one wanted to inhibit him.
Looking serious is better than that.
You might try la-la-la-ing along with the music, and then it would be obvious that you are enjoying yourself.