I just Googled to see who she was...
I think "transvestite donkey witch" is a bit too far, but I love the irony in that episode how they mention "No one would write a book just to make fun of a celebrity" while they're making fun of the same celebrity on the show.
Calling someone ugly is one thing but calling someone a transvestite donkey witch is a little on the mean side. I think maybe Sarah did something to piss off Matt&Trey, or maybe Matt&Trey really really hate Secks and the Shitty, because usually they just "play" with celebrity personas, the don't deliberately insult them just cause.
But the Google images I brought up weren't all that terrible. Photoshop works wonders and the way you wear your hair matters. But searching Sarah Jessica also brought up Dakota Fanning, I had no idea she could be kawaii sometimes.