The reason why CxA supporters make Tifa look like a dumbass is because they have nothing else to show why they're a bad couple. They won't target Cloud, because obviously they need him to be paired up with Aerith, too perfect for the eyes of anyone. Aerith does have that saintly 'essence,' going on with her, whilst they can target Tifa for being dressed up like a slut (when there are outfits in the FF series that are a lot more -quote- sluttier), that she's not good enough for Cloud, and that she's too normal. It's certainly annoying when you're debating with someone stating that Cloud and Aerith have more chemistry and that Tifa's not good enough for Cloud, but fuck, that's the only thing they can throw at us.
I know that there are some Aerith bashers out there as well, but it seems to be that the majority of Tifa bashers overrule the Aerith bashers (at least, Aerith/Tifa bashers who support either CxA or CxT). It's as if Tifa is a more targeted character because she's shown to have more flaws than Aerith. It's not a perfect world, you know.