isn't genuinely felt?

[Eway]

About smile:

"If the eye muscles are not involved in the smile, then it isn't genuinely felt."

Is this saying that the smile itself isn't genuine or people can't feel it's genuine?

[Mariner]

Passive voice is often used to convey ambiguous meanings. In your case, it means both. However, most people would interpret it as "If your eye muscles....then you don't..."

[Ouisch]

Most people can tell whether a smile is sincere or not by the overall facial expression. Usually a true smile also makes the eyes crinkle a bit at the corners. If you smile with just your mouth and not the rest of your face, it looks false.