Epigraphs are like little appetizers to the great main dish of a story. They illuminate important aspects of the story, and they get us headed in the right direction.
The novel has three epigraphs, one in each of the three sections the book is divided into. The epigraph from Level One and Level Three are fictional quotes from Anorak's Almanac, Halliday's guide to his life in the novel. These two quotes—"Being human totally sucks most of the time. Videogames are the only thing that make life bearable." and "Going outside is highly overrated."—both show basically the same thing: life inside the OASIS is preferable to the real world. Or at least, to the real world in 2045.
Level Two has an actual quote, though, from Groucho Marx: "I'm not crazy about reality, but it's still the only place to get a decent meal." This quote seems to be saying that there are some benefits to reality, despite how screwed up it might be. It's just a little strange that it comes in the middle of the novel, between the two quotes from Halliday that contradict it, when Wade still sees absolutely no benefit to reality at all (he never even comments on the quality of his food, decent or otherwise). Wade has no desire to experience any part of reality whatsoever until the very last page.
It also points to the fact that while players are experiencing things on the sensory level in the OASIS, they're not actually being nourished or affected by them. If you eat a strawberry in the OASIS, you're not actually eating a strawberry in the real world—you just think you are.