I imagine Janna's situation to be, up until near the end of the show, one big downward spiral, which probably started with a single misunderstanding over a decade before. Being a weird immigrant kid, she stuck to herself and, as a result, had no one modeling or coaching her in normal behavior in her formative years. So when she unintentionally bullied that cute little mole boy in preschool, she had no idea how to fix it. Her attempts just made things worse every time, until she was old enough for a certain amount of self-awareness.
Seeing how the mole boy pined for her best friend, the most beautiful girl in school, the only little girl nice enough to talk to that weird little immigrant girl years ago, she came to believe she couldn't compete. And so she leaned into the persona she'd unintentionally crafted for herself, harassing Marco more and more. She couldn't help but want to be around him constantly, but if she couldn't be around him as a friend, or something more, she could be always close to him as a nuisance. In this role there was no risk of rejection. She could always be close to Marco on her own terms, without needing permission or approval. And more importantly, without risking being hurt.
Who knew that the extradimensional princess Janna befriended to gain free-roaming access to Marco's house would be the catalyst to improving their relationship? Star was the medium through which Marco could finally see Janna's true colors. And although Starco removed "something more" from the table, Janna appreciated that her relationship with Marco was the best it'd ever been. Yes, friends, the show ended with Starco, but not the story. We all know Starco was merely the gateway to the true ending. The postgame. Manna. Manna could only happen through Starco.
Manna is OTP.