Maybe you can tell from the cover that Not Your Sidekick reads more middle grades than YA. It's got queer characters and kissing, so maybe that's why Lee decided to go YA? The protagonist, Jess, is bisexual, Vietnamese, and the daughter of minor, secret superheroes. She gets a paid internship, working with her crush Abby, and she learns some new secrets about superheroes and their villain nemeses.
Jess has two best friends: Bells, a trans dude, and Emma, who has a crush on a new guy every week. Can you guess what's going on there? ;) There's a lot of easy-to-guessness, which is another reason I think this book would be better for tweens than teens.
My favorite line comes from Abby, "We can't just sit on our asses and not live our lives while we're trying to expose a corrupt government." I feel like that's the world many of us in the US are in right now--rejoining the volleyball team, but keeping our eye on more than one ball being chucked at our heads and the heads of those we love.