Does stress suffered from parents have an effect on the shitheadedness of the kos? These ones seem to have been born okay, despite the torture of their parents...
That's a possibility. Since the stress turns their bean paste sweet, it could be passed on to the children.
Then again, the genetics of it all seems to just be completely random. Yukkuri are just as predisposed to be of any personality type as a human, though much like humans, you can train them away from bad choices.
I've always been curious, as they're creatures of magic and belief: if you have enough yukkuri and humans that believe that shitheads can turn good, would they start turning good?