It's to Pixar's credit that this doesn't work as well as it should. They made the characters faces emotive enough to where even skillful sequence editing can't make them truly menacing in an even an uncanny-valley way. The added music and sounds tried too hard to compensate for this.
It does however raise the question of whether Pixar could do an astounding horror movie by exploiting uncanny valley fears. Something like one of the Chucky movies, but well written. They'd have to relabel it as coming from some subsidiary - an experiment like this could damage the brand.