It's going with the theory that yukkuri shit out their uneasy memories. After all, it's easier to go on with life when all the uneasy memories are forgotten.
Anon experimented by stuffing stalk to the toilet. The baby's growth is supplied with shit paste containing uneasy memories rather than the usual delusional dreams sent from the forehead of their mother.
Shit paste (uneasy memories) bring uneasy feeling to yukkuri, that's why they hate shits and shit-babies. But fortunately this Marisa seems to have a good life, so the only uneasy memories in her shit are about her chuunibyou era. Still enough to made her scream in agony.
1) If you take a Yukkuri stalk and nourish it with 'poo-poo', the resulting child will be shunned by almost every Yukkuri.
2) The reason for this is that Yukkuris memories lie in the paste. The 'memories of being uneasy' are the main ones that get shat out because of Yukkuris' need to be easy.
3) Therefore when a Yukkuri smells a baby yukkuri that came from poo they are reminded of uneasy things and automatically dislike the child.
4) That's in general. But for the specific Marisa they show in the second half of the comic, it's not the case. It doesn't remind people of being uneasy, it just acts very weird. The reason is in the second comic.