Phoebe Maltz explains their difficult position:
I've written here before about the challenges of dealing with negative images of Jewish women in entertainment created by Jewish men. Am I saying Jews control the entertainment industry? No, but Jews do have a disproportionate role in the entertainment-with-a-Jewish-theme-intended-for-a-mainstream-audience industry. Same as all other minorities... We-as-a-society would be a whole lot less forgiving of the Mrs. Broflovski norm if we imagined it was coming from non-Jews.
Add to this the generally-held idea (held, I suspect, by those who've never seen the first two seasons of "Absolutely Fabulous") that women have no sense of humor, and it's clear enough why Jewish women are in a bind if we try to complain (get that - Jewish women complaining?) about the use of representations of our kind as comic relief.
Er, have you examined the character of Mr Broflovski? South Park takes no prisoners. And Sarah Silverman? She kills every stereotype with one bat of the eyelashes.