When working for a big agency in South Florida when I was still young, the agent gave great advice "Nobody wants to see a sober faced musician."
Probably good for a thread of it's own but...not quite true imo. Even as a concert goer I don't need or want everyone on stage putting on their "face", especially if the musician has something to say through his/her instrument. The keyboardist is probably the last member that needs the fake smile, gaze into the distance, or worse. Even during their solo moment in the spotlight I rather see them concentrating on their craft and giving it their all. No need for any visual entertainment- that's what the lead singer, guitarist and back-up singers are for.
It's the thing that turned me off from gigging long ago. I get that everyone is an entertainer and certain band situations warrant it, but it's often overdone and comes across as contrived. Let there be visual focus.
We have to remember that we listen with musicians' ears. Most of us could care less if the player is attractive, smiling, nicely dressed, or anything other than the music.
But when playing to the general audience, presentation is as important as the music.
Phony, pasted on smiles don't make it. But remember we are PLAYing music, and PLAY is the key word. If we are PLAYing we should be having a good time, and if we're having a good time, it should show with a happy face. There will be a few in the audience who come to actually listen intently to the music, so it's best that you play well too. It's a package deal.
At least that is the way I see it. On stage since 1964 it's worked for me. And I don't have to fake the smile, because gigging is the most fun I can have with my clothes on.
Of course, as always YMMV - there is no one correct answer.