In the early 90's, Pizza Hut sought to attract younger customers, particularly tweens and teens. The gimmick they hit on turned out to be a radical revamp of an old Saturday Night Live skit.
The Pizza Head Show was a series of ads for Pizza Hut that ran from 1993-7, from Mr. Bill creator Walter Williams. Shoot, Pizza Head even sounds like Mr. Bill. Sixteen commercials, mostly for promotional giveaways, were produced over the course of four years. It so happens that an enterprising YouTube poster collected them all.
I think now you know where the creators of Uncle Grandpa got the idea for that sentient slice of pizza.......