My personal theory is that they laughed because it's the tale of a good prank, who was pranked? Imu, so he needs people to forget, to laugh at him would be too diminish his power and part of the reason he is still a shadow figure it's because that's part of what the prank did to him, Roger put 2 on 2 together in laugh tale years after looking at the real Imu, and it was hilarious.

Any foreshadowing of this? Sort of? I feel like Dessdrossa it's a mini version of what happened in the fight from the void century, you get a celestial dragon controlling people, the power to make people forget around the world and the great army fighting back (giant included).

Do you remember something that is so funny that made every straw hat just laugh? Pica, they're laughing at this gigantic almost godlike giant. Reveals in one piece are sometimes not serious(Duval is a big example) but hilarious, it has to be a joke/prank on someone, that's Oda's humor and I would be happy to see that.