its own WMG page attests, Gumball's universe is an idealized vision of what the founders of Ink City are trying to create — a world giving animated characters from all walks of life and realities the chance to build a wonderful world together, making friends and forging bonds that never would have gotten the chance to exist otherwise. Unfortunately, the residents aren't getting along quite as much as the founders had hoped...
The Mayor and The Painted Lady were muns themselves, possibly brother and sister.They just got sucked in through some kinda reality warp into a blank universe, in which they took on the aspects of Ink and Paint and set about creating their own city, a la limbo in Inception. Once they built it, though, they realized it was empty... and created the Ink Fountain in order to draw animated characters into their world.
Ink City is set inside a Cuckoo Nest.The characters inside aren't the real ones; they're ink-based clones. The originals are being held elsewhere, relatively safe — at least, in the sense that they aren't in any physical danger. Their minds, however, are subjected to all the threats of City life. They provide the template each resident needs in order to exist. This simulation allows those responsible for their imprisionment to monitor the experiment secure in the belief that they aren't really harming anyone: after all, Ink Clones Are Expendable, right?
Trevor really WILL become Zuul.As well as an even more massive Tool. And he will, in fact, proceed to find Lord Hamster, just as he declared to the Muns. This will end poorly enough that the entire Trolling incident will become much Harsher in Hindsight.