It's not in "good fun", it's and active an ongoing sex based smear campaign.
I don't think this is sexist alone. They're playing into people's worst perceptions of Palin -- unqualified, out of her league, dumb, poorly-spoken, "folksy bimbo," etc. I don't think she's dumb, or a bimbo. And I suppose some of the stereotyping is sex-related. But it's not as if they're going "she's a woman, so she's an idiot." Do you remember many informed people calling Clinton stupid? A few, as there always are, but not many.
Of course, her stereotypes were a lot more on the sexist side.
