Look, the writers may or may not make any certain character Jewish or not, but the entire show is based on certain types of people in the Jewish culture, or you might say, how certain types of people are categorized/classified in Euro-Jewish culture.
It's called "schlemiel-schlamazel." Kramer is the happy-go-lucky schlamzel, who, being the Lucky Gentile (non-Jew), falls in manure and comes out smelling like the proverbial roses, while Jerry, Elaine and George are the schlemiels (Jews!!), who can never be happy and just live; they are always striving in life, often unlucky, and cannot believe how the schlamazel does it. (Does George's observation about Kramer ring true or not? I quote George speaking of Kramer: "He gets money without working; he gets sex without dating; he something or other, etc; his whole life is a fantasy camp!")
Or is it the other way around? Is the schlemiel the Lucky Gentile? I disremember. You may remember this schlemiel-schlamazel refrain from the intro to Laverne & Shirley. One was the schlemiel, the other schlamazel. I don't know which; I never really watched it much, but that raucous show intro music could always be heard everywhere, it seemed, a couple of decades ago.
This was the subject of a paper in a scholarly journal in the television area (yeah, there are scholarly journals with TV as the focus, believe it or not).
Anyway, they may or may not be Jewish by the script, but it *is* a "Jewish thing." Period.
And, btw, I'm not Jewish.
Randy