This is a 'chicken-and-egg' debate which can never be answered.

If you sing, you will discover that changing your expression will change the tone of your voice. So do we:

a. sing lyrics and make an expression to match, or do we

b. express emotion with the face and let the voice follow suit?

The answer is that they go together and also that there will be variation among individuals.

For those who are bothered by facial expressions, my advice is to stop staring at the performer, look at something else, and listen to the music. Sound is what's it's all about, anyhow.