After the confusing hoverboards, let's talk about a simple symbol, the pendant that Cable gives to Tally to track her to the Smoke. And what makes the heart-shaped pendant so simple is that Tally tells us what it symbolizes: "It felt as if everyone in the Smoke could see what the necklace really was: a symbol of her treachery" (29.4). Really, we can't improve much on Tally's commentary here. It is a symbol of her treachery.
But you didn't think it was going to be that easy, did you? The pendant may be a symbol of her treachery, but why is it heart-shaped? Well, probably because it symbolizes how divided Tally's heart is: she wants to get back to the city and be made pretty and party with Peris; but she also learns to love the Smoke and, ahem, David.
Imagine how different this would seem if her pendant were, say, lung-shaped.