I think those early meetings between potential new family members can be very tricky. Yes, Daphne should be herself and show her sense of humor that her family knows so well, so she and Clarissa can get to know each other. But on the other hand, I think that particular joke is of a questionable type that Daphne should probably have saved for a bit later in the "getting to know you" process.
Fundamentally, though, if Charles says it was a joke, I think Clarissa should try to accept that (and maybe she has by now). If she doesn't trust Charles's assessment of his own family members, I think they have bigger issues. But of course that does happen sometimes, with the new spouse being truly hurt by something the in-laws said or did, that is brushed off as "a joke" or "normal" by their SO, who's used to that kind of (bad) behavior. I think this single incident is not enough for Clarissa to be worrying about, though.