The impression I'm getting is that, for a substantial number of people, crossing human with something other is an automatically troubling concept/visual, especially if that something is an animal - it falls under the broad, subjective umbrella of the 'uncanny valley'. The Cats movie seems to have hit that particular nerve particularly hard, and in our reactive, feedback-loop world of medias social and mass, those reactions are inevitably magnified.
That the stage costumes could generate a similar response in some is perfectly understandable.
I don't think that this is what uncanny valley actually means, but I agree with everything else in your post.