Someone's been getting their dating tips from Deliverance! Despite what man-raping yokel inbreds may tell you, I think you mean "a pretty face." The only people that focus solely on the mouth part of the face are serial killers and dental assistants, neither of whom you want to get involved with. BURN, dentistry.
I'm of the opinion that a guy makes his initial snap judgment of whether to even talk to you based on your pretty (or unpretty) face, but that will only get you so far. Personality, or more specifically the dynamics between the two of you, are the infrastructure that keep your relationship from collapsing.
And even better, personality has a cumulative effect; you really do fall more deeply in love with your mate (assuming things are going well) as time goes on. A pretty mouth twenty years from now probably isn't going to do the trick.
Of course, most guys take many years to figure this out, so until then, just botox, purge, and bikini wax until you can sex-trap a rich dude and get in a wicked knife fight with his estranged step-daughter.