I'm not at all sure what the problem is; when I first started seeing live ballet regularly, one of the few things that I found distracting was the grinning rictus that appeared on many faces - and sometimes the whole corps (much as described by Merrill Ashley, above). It must have been choregraphed, and was awful, robotic and distracting. Happily, that seems much rarer now.
I'd suggest that teaching dancers to focus effort on facial expression is an error; for a start, they have more important things to do, also attempting a half-smile, 'to disguise effort' could end up an almost anything after a lot of effort, and a need to concentrate on what comes next.
I'd much rather see - as others have suggested - a calm, in control dancer, who does not feel they have dance with their face in any way at all, though, hopefully, they'll play down any pain or difficulty - and, as actors, they'll remain in character.
In some comedy ballets, there is a specific interaction with the audience at times - Elite Syncopations comes to mind - where the audience is 'in on the act'.
I'm happy to say that I've never ever seen the malaise described here, though I'm equally happy to say that I've seen some beautiful smiles from dancers, usually in response to audience appreciation.
Is there a problem, or it it just a reviewer with writers block?
That has to be just plain spite, doesn't it?
But has it something to do with the specific character in the specific ballet - way over the head of our critic?