I wouldn't say that smiling is a part of mania, so I wouldn't say that it is legitimate criteria either. Everyone behaves differently. For me, personally, I start laughing and smiling because I am so happy/elated, but I also never, ever smile in general because of my flat affect. The smiling is just an artifact of feeling so happy and revved up (i.e., it is not a direct symptom of mania, but rather, a symptom caused by elation). I am sure people do smile when manic, but they might also smile even when euthymic. In that case, I would say no, smiling is not a symptom.