Sienfeld portrays all American Women as whores, and plays as though Jews will renounce Judiasm and marry their "shiksa":

1st clip states shiksa is simply a "non-Jewish woman",
2nd clip includes more scenes

Is Shiksa always a perjorative or a foul oath?

Yes, it comes from "detested" thing in Jewish Culture:
This applied to a woman is calling her a whore, or worse really it is calling her worthless and they use the word against women they would make worthless by whoring them.

The part at the end of the video where Elaine is jamming a banana into her mouth while Jerry acts incapable of resisting proposing marriage to her is a reference to this quote from a book many Jews have read, an Easter egg or hidden gift to the Jewish audience watching the show to compound the mockery:

Doctor, I had never had anybody like her in my life, she was the fulfillment of my most lascivious adolescent dreams- but marry her, can she be serious? You see, for all her preening and perfumes, she has a very low opinion of herself, and simultaneously- and here is the source of much of our trouble-a ridiculously high opinion of me. And simultaneously, a very low opinion of me! She is one confused Monkey, and, I'm afraid, not too very bright.
― Philip Roth, Portnoy's Complaint

The Jews are laughing because they would never actually act like Jerry or the other Jews, but the show is intended to cause American women to think they are desired by Jews so Jews can whore them, denigrating them to uproot and destroy the dignity of such folk, as they call them shiksa (or their word for "whore"), all while the American women feel it has a positive connation.