Facial expression of course is part of it. I can't imagine how somebody who skates around with a blank face the whole time could be conceived as having strong performance skills. But performance is storytelling more generally, and you can tell a story with your arms and legs and chest as well as your face. Storytelling is emotional range and saying something even in quiet moments. I don't see any storytelling or performance in Zagitova's skating, just a lot of awkward kicking and arm-waiving in various directions.
She's certainly not the only skater with this problem.
It's true Alina doesn't give a lot of face during her performance, but she looks appropriately intense/dramatic during Black Swan, and appropriately flirtatious/happy in Don Quixote. I would hardly call her stone-faced.