This nuance in a deliriously unsubtle show filled with some of the grossest gags around is why Big Mouth rises above its most puerile instincts (and there are a lot of those). The physical manifestation of emotions is both insanely clever, and a way to dig into the psychological torments of puberty in a way few other pieces of entertainment have.
In the last episode of the season, Nick, Andrew, and Jessi take a magical journey to where the emotional sausage is made by jumping through a dimensional portal left open in Nick's bedroom by the incompetent Hormone Monster Tyler (John Gemberling). This parallel universe is a grand office building, and when they head to the Department of Puberty, they encounter a whole range of Hormone Monsters, including Gavin (Bobby Cannavale), an intimidating mass of testosterone. Meanwhile, in a conference room, the various beasts representing all the signals flaring in a burgeoning teen girl's head are battling over the soul of Jessi, who has been sent down a path of mild truancy by her Monstress Connie in the wake of her parents' divorce. There's an Intellect Sphinx, an Ambition Gremlin, and an Anxiety Armadillo, all worrying that Jessi's potential is slipping away. The Sphinx invites the Depression Kitty (Jean Smart) to take over. The smooth-voiced Kitty offers a false sense of security, lulling Jessi with inaction and melted ice cream. Jessi, hearing the voice of her friends, understands that she needs help. Still, I would guess we haven't seen the last of Kitty; depression doesn't go away that easily.
The Kitty -- combined with the Sphinx, the Wizard, and the rest -- reveal that Big Mouth all along has been a more disgusting version of Inside Out, the Pixar movie where a girl's emotions were represented as a series of adorable blobs battling it out. But where Inside Out was concerned with letting go of childhood memories (RIP Bing Bong) as a path to adulthood, Big Mouth is focused on the nascent mature anxieties that flood the cortex. It's telling that, at this point, Jessi is the target for monsters that aren't just hormonal. It's often more complicated for women.
When I spoke with Kroll over the summer, he mentioned that he doesn't want to keep Big Mouth locked in the same time period forever, and indeed, there will be plenty more work for the Shame Wizard and the Depression Kitty as the characters grow up. With an entire dimension of emotional consciousness in play now, probably some other fantastical beings will pop up too.