Does the book, Lost in yonkers have the theme "reality vs. illusion?" lol

Yes, but I wouldn't say that is its primary theme.

By that I mean, other themes are more important. Look at how hard the family works to stay together, and what they have to do to survive. Those get more emphasis in the play.

That said, yes, there are a number of times when that theme or a related theme (like lying, or distance from reality) is emphasized. For example, the grandmother keeps money hidden, so it is not visible. When others search for it, they lie about it. Louie is double-crossing other criminals. And of course, Bella daydreams and deludes herself throughout the play.