While I can not claim that I KNOW “anything” about this, I can tell you that “Fresca” does NOT mean prostitute in México.
Actually, the slang-used term, is the same as in American English. “Fresca” means “fresh”, and you can use the word to refer to a soda that is “fresh”, or to a person that is “a fresh”.
Now, it is my WAG that when they decided for the word FRESCA they intend to do a little word-play, since the word we use in México for a soda, or pop, is “refresco” (“refreshment” [masculine, in this case]). So the name for this product rimes with “refresca” (refreshes), which is actually part of the publicity slogan they use.
I have never heard anyone use the word “Fresca” as slang for ‘prostitute’, and I’m mexican.