Jeannette gets her gift of metaphor from her mother. When gazing at a Joshua tree in the desert, for example, Mom says, "It's the Joshua Tree's struggle that gives it its beauty" (2.8.32). That's a good moral for this book, in which Jeannette's story of struggle becomes a beautiful tale of triumph. Jeannette herself is a like a Joshua tree—even if Mom herself isn't able to quite understand that she's creating the struggles Jeannette has to overcome.
The Joshua tree also shows us Mom's ability to spin positive out of anything. As Jeannette says, "We were always supposed to pretend our life was one long and incredibly fun adventure" (2.17.26). Thinking of everything as an adventure is a way to keep the kids thinking positive. But pretending things are okay for so long is pretty much just inventing metaphor and fantasy—the kids come to realize that their lives are actually pretty bad, and Mom is just creating a make-believe world for them. Perhaps that's one reason Jeannette has such a strong imagination.