I think some acting coach must have taught her that full open mouth (smile?).
I believe it is/was an attempt to make her seem congenial, likable, and human.
That dumb look/head move she gave when the balloons came down at the DNC is an offshoot of that face.
I find it creepy and entirely fake. She does it with the emotion of a handshake and she uses it constantly.
It is cartoonish and off putting, in my opinion.
Anyway, I totally agree with you on the phony open-mouthed smile. So contrived and fake. Also the way she points randomly into the audience with that expression of surprise, like she just noticed a long-lost friend.
Nothing genuine about her whatsoever.