Discussed: "You know Arisu from the next class? After she caught Takashi cheating with Chikako, her eyes become lifeless. Scary!"
Conversed: MST3000 comments that "Arisu should REALLY get some visine about now".
Deconstructed: After the reveal, Arisu's eyes become lifeless, but it's actually a manifested physical condition of serious mental breakdown. And as she spiral downward into insanity, her eyes become more and more lifeless.
Reconstructed: After therapy and proper medication assisting in a beneficial spiral where she regains her mental health, Arisu's eyes are shown to become more lively over time (or in specific circumstances after therapy, like the healing power of kittens)
Played For Laughs: Jamememba sees Arisu's eyes and attempts to use an eydropper to cheer her up.
Played For Drama: The characters know that Arisu's lifeless eyes are indicator of her nearing snap, so they do their best to prevent it from happening. Arisu is a known Yandere.