Susan is joking, though this conversation is somewhat out of character for both of them. Not entirely, but it goes to an unnecessarily weird place in the end. They're still developing their personalities at this point, however, so shut my mouth I guess.
Still, Susan making a "fully functional" joke? I guess as a Star Trek fan, she felt someone had to.
That said, I'm pretty sure the entire point of this comic is actually to show that Nanase is with them.