It's a joke! You probably figured out sometime in elementary school that "God" spelled backwards is "dog." By playing on that, this title refers to the phrase "there is no God" that appears in graffiti at the end of chapter 26.
But it's not just for the lulz. Here are a few more things to think about:
(1) It's a pretty funny (and no funny ha-ha) title for a book that is totally about God. It's kind of like saying, "There is a God—he's just not very good at it."
(2) There are no dogs in the whole book. Go on, check! There are capybaras, Ecks, and lizards, but no dogs.
(3) Bob is dyslexic, and people with dyslexia sometimes spell words backwards. "In other words, "there is no dog" is something Bob might say about himself.
How do all these add up? The title is just one more way that Rosoff takes what could be a funny and/ or potentially offensive book and asks you to think just a little harder about it.