I never judged off one performance or one video, it never judged at all. I don't think she never smiles nor do I think she has a stick up her butt xD; But like you said, there are times where she's concentrating or trying to be cool or whatever, and she doesn't smile... Those kinds of videos are the ones I've been exposed to mostly =/ And a few in which she's been smiling her ass off xD; But when the balance is uneven according to a certain individual's perspective, it can give the off the feeling that she does not smile... that's all
I'll try and rewatch some of the concerts I have of her (cause I haven't seen them all the way through).
I think she has a goregeous smile