In order to understand what Hope is thinking here, put yourself in her shoes. You are a kid who is from New York City. And now here you are driving in to this town in Wisconsin. The sign says "Population 5,492." What would you be thinking? I believe that many kids in this situation would be thinking that they had been exiled to the middle of nowhere. That is exactly the opinion that is shown by the quote you cite.
There are two aspects to the quote. First, "no subways." This shows that Hope is contrasting the smallness of Mulhoney with the size and bustle of New York. She's thinking she's in a tiny place now. Second, "no sushi." This shows her contrasting New York's cosmopolitan atmosphere, with people and things from everywhere in the world, with this little place where you can't even get sushi.
Because she's stuck in this tiny little place that is so backwards that it doesn't even have sushi, she sinks in her seat. This shows us her mental state in a physical way -- she is mentally drooping and slumping because her first opinion of Mulhoney is that it is the middle of nowhere.