Kaylee understands that Mal is very upset and well on his way to drunk, which is why she manfully restrains laughter.

And why it's gently that she says, "Yeah, but captain, you're forgettin' one thing."

Beat.

"If they can make her Barman, then she can make somebody else Barman, too. If that happens, and Mike does a runner. Which he wouldn't. But if he did. It'd be fine."