So at the first moment the "little one" isn't cute the mother panickes and abandon it?
This comic only adds points to my theory of the yukkuri shallow nature and standar, aside of the "cuteness" obsession.....even when they aren't cute on non yukkuri standars.
Or, maybe, that's all Reimu can do? Look at what its options are. It can't fix the eye and anything it does would likely make things worse. The Marisa's not going to calm down and sooner or later will get angry and do something rash (likely leading to further injury or death).
So Reimu can either stick around and wait for the inevitable meltdown and death, kill its ko or leave. One of those is less uneasy than the others; of course the average yukkuri is going to pick that. Assuming we take things at face value and Reimu abandoned Marisa so Nature can take it's course. Maybe it went off to find help; we don't know. The focus on Marisa removes any insight as to its choices.
Or we could just dismiss it as another unworthy shitheaded yukkuri.
The mother could always, cut out the clearly beyond recovery eye and try to patch up the best she can, yukkuris, do know, how to heal to some degree their fellows, not in the way humans can heal yukkuris, but at least enough for this little one, to just become a 1 eyed yukkuri that still could live an averagely normal life if she worked hard to take it easy.
Loosing an eye is a very traumatic experience, but, in many animals, at least land based, it shouldn't be a definitive factor for the "it's beyond salvation, let it die as nature's law dictate", many animals and monster fauna may lose 1 eye and still go on living without much problem.
Yet this mother, seems to had panicked and left her lil one alone, not even going "i'll find help!", just plainly left it there, to it's own luck, not even taking out the damaged eye properly.
So yeah, dunno if this reimu is shithead or not, but clearly i'll not aprove of her and consider this outcome "acceptable".