Surprisingly enough, a series about the bombing of Hiroshima and its aftermath manages to have quite a bit of humorous scenes in it.

* Gen and Shinji fighting over a sweet potato early in the film... until Eiko grabs it away and scolds the boys for fighting over it when food is scarce and their pregnant mother has to eat for two.
* Nearly half the things Ryuta says, especially in the manga.
* In a later volume of the manga, Ryuta and another boy escape from prison and need disguises. They pull a GoneSwimmingClothesStolen on a boy and girl they meet on the road to Hiroshima. The other escaped prisoner can only fit into the boy's clothes. Guess who's stuck wearing a frilly dress and a hat with bows?
* After the horrifying bombing scene we see Gen emerging from rubble. What his reaction? Fear? Sadness? No. A deadpan "wow, I guess they really did bomb us after all". Weak dialogue? Or a moment of pitch black humor? You decide.