Casual / colloquial, slightly childish or rustic. Most adults today say 亲 alone (亲一下) or the borrowed 接吻 for the formal/literary register. 亲嘴 is fine in family/dialect speech but rare in print.
mistakes
亲 by itself can be a shorter, softer way to say "kiss". 我亲了她 is more romantic-sounding than 我亲嘴了她 (which does not even take an object cleanly). 亲嘴 stays intransitive; for who-kisses-whom, switch to 亲.
