Inklingification - *stab*

Cyn is most certainly NOT meant to look appealing. She's a cannibalistic incestuous stalker wearing the skin of the one human trying to keep her out of slavery. It's even shown that her "cute" text-to-speech-impediment causes her endless frustration at being unable to properly emote like her brother

She's popular for the reasons the FNaF animatronics are popular: scary, but without being repulsive