It was hard to understand you were supposed to be dead, when you felt and in fact were, very much alive. A possible head injury certainly didn't help either. Neither of these things would stop Wash from not taking her completely seriously.
"That's right. I'm supposed to be dead." He smirked, though that expression drifted into a hazy, ear-to-ear smile post-kiss. Carefully he slid his legs off the side of the table, bracing his arms on either side of him before moving to stand. "There is some good news regarding that, besides the fact that I'm not dead. I also do not appear to be a zombie. There's no immediate hunger for the brains of the living --then again that all could change." Even if he understood her view of things completely, he still would have probably gone for the joke. So what if he'd been dead? He wasn't now and that was something to be celebrated. Hence, joked about.