What is the moral of the story in To All the Boys I've Loved Before?

I suppose the main moral of this story is that honesty is always the best policy. Most of the many misunderstandings that arise during the course of the book are related to a lack of honesty. For instance, Lara Jean lies about the true nature of her feelings for Peter and tells him to his face that she's dating someone else. For his part, Peter does nothing to challenge the unfounded rumor that he and Lara Jean had had sex during the school ski trip.
It's this inability and unwillingness to face up to the truth of their mutual feelings that leads both Lara Jean and Peter into all kinds of trouble later on. Thankfully, they are eventually reconciled. But this would've happened a whole lot sooner had they both been open and honest with each other from the start.