You seem to confuse "someone likes you" with "someone finds you attractive". Maybe someone smiles at you just because you seem like a nice person. I would even argue that this happens much more frequently than someone smiling at you because they find you attractive.
Well, this is aimed at straight dudes in any case, so in this mindset "girls" (!!!) only either are for sex or unimportant.
Nice, we can also assume that P(she smiles at you | she likes you) is approximately 1, simplifying to only 2 variables, and also substitute P(you are likable) for P(she likes you) to remove all unknowns.
and also substitute P(you are likable) for P(she likes you)
That seems like a pretty wild leap of logic. Being likeable in general isn't a substitute for a specific person liking you. Though there's probably a correlation related to your overall "likeability score".