It's a bit of a shame - the artwork is nice and expressive, I love the comedy and the fluffiness of it, but I just can't get over how it's a plot point that Ruu used to get straight 100s in elementary school (and therefore clearly can't be actually stupid) but turned into a complete spaz over the span of three years. It's supposed to be endearing, but her utter lack of maturity is awakening fatherly instincts more than anything, and as a father I would be horrified. Just what kind of home life does she have that failed her so completely? This isn't even arrested development, she's actively regressed to being less mature than her elementary school self.
Kinda implies that her "gyaru" interests actively made her stupider, too. Which doesn't quite gel with the "gyaru acceptance" angle. It rings hollow when the manga implies that she's fine as she is right now. I understand the sentiment... But she's clearly not fine! The only light on the horizon is that she does make that jump in the most recent exams, and there's still plenty of high school left. If this is going in the direction of her clawing her way back from the brink only to overtake Tadashi for #1 at the very end and embarking on a bright academic future, and marrying him once they're at eye level rather than him being her adult caretaker, I'll be mollified.