In reply to:

So - is this a legitimate use of this word, or did our cartoon heroes just adopt it for their own uses?

The confound and astonish sense of transmogrify is the secondary meaning. The primary meaning is as used by Mr Waterson: to change into a different shape or form, especially one which is fantastic or bizzare. (American Heritage) Webster's says its origin is unknown. I wonder at the redundancy of meaning among transmogrify, transmute, and transform. Now, where is that philosopher's stone???