This is a smack/slap with the hand, not with the lips - that would require a different word.

For reference, I have lived in California, United States my whole life. I immediately understood that she smacked him (hit him) on the cheek. I’ve never heard someone referring to a kiss as a “smack”, (I’ve heard it referred to as a “smooch”)... However, someone can “smack their lips”, which means they made a noise with their lips, but I don’t get an image of a loud sound from a kiss when I hear that word; it makes me think of a little kid who was enjoying pizza or some junk food or something... Hope that helps.

She gave him a slap. The other option I think you're thinking of is a smacker, which is a slang term for a loud kiss (or in British English, smackers can sometimes be used to refer to our currency, i.e. "one thousand smackers" = £1,000)

To me, it sounds like she hit him in the face. I am from Texas.