User blog comment:GravityMan/Literature vs History 7: Cat in the Hat vs Bill Cipher/@comment-4920659-20160221031031/@comment-24322797-20160223130741

Bill never broke the fourth wall directly in the show, but Alex Hirsch had made a lot of jokes about Bill existing in our reality as well. iirc he tweeted a picture captioned "Arguing with my boss" where it was his shadow and Cipher's shadow, and he also did a AMA on Reddit as Cipher where he answered questions that relate to todays society

apparently all eye of providences in the world act as "interdimensional peepholes" or something like that

so no he hasn't directly broken the fourth wall in-show but he's acted outside of it