But I think it’s interesting if McKay wasn’t leaping ahead to “nice,” but rather to “fair,” which would actually make his response make more sense.

I totally agree with you. In fact, I assumed Carson said fair and was surprised to learn that *I* completed the sentence that way, not Carson. And I don't think Carson would bother correcting Rodney's niceness in a situation like that, but I think he would take the time to call him on the unfairness of the comparison and the assignment of blame.