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.