Check out how the book ends:
When I wake up I swear I can still taste the mustard on my tongue. It doesn't take me too long to figure out what the dream means. (15.274)
Hmm… something tells us Mia doesn't want hot dogs. In fact, she's talking in code. She's still getting over the loss Mango when she learns that he has a litter of kittens with the next-door neighbor's cat. The littlest one of the bunch looks exactly like Mango, and she can see a mustard color when he yawns.
Mia is clueing us in here to the fact that she's going to get the kitten. She might not feel totally ready for him yet, but her dream encourages her to go out and give Mango's son a proper home. We've just been through the ringer with Mia, dealing with her cat dying, so we're glad to see a happy ending for her. Even though we don't get to see it, we know Mia's going to be happy again with her new fur ball.