One of the few, yet common, criticisms levied at the 2017 Made in Abyss television series was at animator Kou Yoshinari’s creature designs and animation. When Yoshinari was given creative control of how the creatures of the Abyss would be designed, he purposefully made them visually-jarring and otherworldly. They’re animated as an other, with blurred lines that distinguish them from sharply-detailed backgrounds of the Abyss itself.
Alongside the citizens of Orth and cave raiders of the Abyss, we as viewers know little about this gaping maw other than what the series tells us. The series’ visual direction makes us part-time travelers with Reg and Riko, and part-time voyeurs, watching them from a distance with a calculating eye (like the monsters, the Abyss which is a character all its own, or high-level cave raiders like Bondrewd who have eyes everywhere).
The monsters aren’t animated badly, they’re unnatural by design.