Casual / colloquial — slightly childish or rustic. Most adults today say 亲 (qīn) alone ("亲一下") or the borrowed 接吻 (jiēwěn) 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 doesn't even take an object cleanly). 亲嘴 stays intransitive; for who-kisses-whom, switch to 亲.
