I agree with you that Rick and Morty isn't a nihilist show, and neither are any of the characters nihilists. Rick is deeply cynical and maybe a philosophical pessimist, and he's definitely a moral relativist, but I wouldn't call him nihilist, no.

Your first paragraph is a really good set of examples, so I can see now how the characters can be perceived as nihilists. But my personal conception of how nihilism would operate in real life is that it would be intrinsically erratic and dysfunctional, with the nihilist continually struggling against an inherent lack of motivation and a biological drive for self-preservation. In that sense I consider nihilism to the functional equivalent of absurdism and pessimism. Absurdism has essentially the same core tenets as nihilism, but its literature reacts to a lack of intrinsic meaning with flippancy and a declaration that people should ignore that and continually strive for meaning anyway (which is inherent dissonant, but basically don't think about it and live on). From there, nihilism appears to be, in my opinion, what happens when someone encounters the dissonance of 1) no intrinsic motivation anymore, brought about by intellectually grappling with existential despair, and 2) instead of leaving that state, whether through traditional existentialism/religion (Kierkegaard) or through tucking it away and forgetting about it, lives on with it and doesn't commit suicide out of biological necessity.

In contrast I think the examples you posed about Seinfeld's characters are more like deep self-absorption or narcissism, and maybe outright sociopathy. In a certain sense it seems like a much more implicit form of the unsettling indifference portrayed in a Brett Easton Ellis novel.

I think maybe the problem here is that we're labeling characters but really should be talking about the worldview of the show, revealed through the characters.

To that point, I'd observe that the show's most unhinged character is also the one with the most humanity and alertness to the world around him. What is the show trying to say about life by suggesting that you'd have to be like Kramer to escape the gravity well of self-absorption around Elaine, George, and, most of all Jerry, the character you're most meant to identify with?

Kramer chides the group for poor behavior or passivity from time to time. He helps people with their problems. I'd dispute that he's nihilist.

That's true, but that's part of the joke: only the weirdest, most uncomfortable of the 4 characters is allowed any humanity.

You'd be right to dispute he's a nihilist. Out of all the characters, he's probably the least aware of the nature of his own existence, yet he's also the most well-adjusted to a life motivated by money and poontang. The character could still be seen in a nihilistic light because, despite his own moralism, he frequently contradicts himself and often ends up in a better place for it. If one were to read too far into what the writers were intending with Kramer, they might assume that he is meant to be a commentary on morality since his morals don't consistently serve him or the other characters, and Jerry can easily spot contradictions in Kramer's reasoning.

Seinfeld's motto guiding the series was "no hugging, no learning". It was set up to be a foil to the sitcoms of the day, which were morally authoritative and each episode was there to teach us a lesson.

There's only one somewhat-serious moment in the entire series, and it's when Jerry tells Elaine he doesn't want to get back together.

It's really brilliant and stands the test of time so well. But yes, definitely nihilistic.

Yes, you hit the nail on the head. I was referring to the characters.