Good grief. While Sazanami's Muppo is just cute and huggable, Yamato Nadeshiko's is just downright scary and creepy. I don't know why, she's just....creepy
Might be the color, the passive aggressiveness or the fact that Hoppo-chan might be sucking up all of Muppo's cutenes, leaving only creepiness.
I think it's the coloring that's making Muppo look creepy.
Ghost grey? That's part of it. The other part is their relative sizes - Muppo is a head taller in all the other art done by Yamato Nadeshiko. That shoves Muppo past a childlike height, while in Sazanami's art the height relative to other things didn't make Muppo a giant in comparison to her surroundings.