I disagree.
Its not about her not meeting our expectations, its about her turning into a bad person in adulthood. This isnt about "oh man why isnt she the Earth Queen and working eyes." This isnt about perfection, its genuinely disliking a dislikable character. And the disappointment that it was a character we all loved before...Some more than others. Looking at you foot-fetish Child Toph creepers.
How is she turning into a "bad person"? There is nothing suggesting that, nor is bad parenting (and I don't necessarily blame her for everything, as we don't know the details) an equivalent to a "bad person". Not everyone is fit to be a parent, nor did Toph have the best influences - Katara had lost her mother, Zuko parents weren't exactly "perfect" either, each of the ATLA group had issues and flaws - so it's going to be apparent when they're adults as well, none of them are exceptions to the rule, but that doesn't justify the idea of them being "bad", flawed parenting... sure - a bad person in general? No.
And sometimes, no matter the parenting style - a child will act out. It happens.