This place can make people a little crazy sometimes, but I have to agree with Kara up there.
Laughing doesn't necessarily mean they aren't taking things seriously. Sometimes it's either laugh or cry before going into a fight, or even during the fight, and I'd guess they'd rather show you a brave, crazy-seeming face than tears and pain and fear. Some of them had a pretty big adjustment to make being formerly dead and some people just deal better in stressful situations by cracking jokes and seeming like they aren't taking things seriously, when in fact, they're probably the ones taking things the most seriously inside.
But I say that knowing someone who is very much like that.
Granted, that cartoon is seared into my mind now and I can't blink anymore without seeing it, so we'd all have been better off if they'd come across the tamer things that artist drew, but maybe it's helping them, even if it's upsetting you.
You should tell them you feel like this. They're the only ones who can explain why they're behaving that way, after all.
... And that was really long. I'm Sarah, by the way. Now that I've rambled at you and all.