Feels like that is the point. They made an incredibly sexy chemist girlfriend, so you know that's what she is really like.
And then turned her into a 9-year-old diaper wearing idiot, dating a 16 year old. who is also an easy plot device or deus-ex-machina. That's all sorts of f'd up, but remember, she's actually just a highschool student right?
That's what I kinda love about the show, not many who can pull this stuff off.
Even though I hate diapers, underage stuff, loli's and all that what would usually land you in jail. But the rest of the manga makes up for it.