Personally, I don't think it matters what they were laughing at.I thought from the one cab confessional that they were likely to laugh at something, anything, just to see how much they could escalate the pissed offness.
Step 1. Tell everyone your version of the story where you are totally innocent and everything is always someone esle's fault.
Step 2. Find the most obnoxious reaction you can to the other people melting down over being cast as the villian which usually is laughing.
Step 3. The more they melt down, the more you laugh.
I think there may be some part that was about how Luke looks when he gets passionate when he is signing angrily and trying to vocalize what he is feeling. I think, however, they would have laughed had he been a hearing person having a fit.
I think they knew full well that they could make anything worse with the snide but pretending to be oh so innocent laughing. I have a feeling they've escalated things that way before and know exactly how to play it for maximum effect.
It was obvious neither side had much interest in working things out, but I think K&J were much more invested in firing up the Mom and Luke than the other way around.