Well, to a certain extent all the Buffy characters are based on broad stereotypes to begin with, such as vapid valley girl, computer nerd, class clown/joker, the stuffy english guy and the spoilt cheerleader-princess type.
We only see Tara's family for one episode so its hard for them to be fleshed out as characters, so they remain the one-note character's that you outlined.
Having said that we only see Willow's Mom for one episode and she seemed far more fleshed out than Tara's folks. (and also managed to explain where much of Willow's insecurities come from...)
Add to that the lack of characters that come from the southern states/heartland on the show, and its hard not to see Tara's family as representative of 'those sorts of people'- backward, patriarchal, insular and uneducated.
They seem to represent (incorrectly) what a lot of people think of folks from rural, 'flyover states'.
Also remember that that Joss himself, a big Democratic supporter, wrote the episode, and it would be easy (though not nessesarily correct) for someone to assume some lazy predjudice at work here.