Ron, in canon, wasn't much less bright than Harry: they were both fair-to-mediocre students who bumbled through adventures mostly on the strength of luck, chutzpah, Hermione, and the beneficence of older and more experienced characters. Ron being a non-viewpoint character, though, we didn't have as clear a view on the motives behind his failings, and so they came off as less explicable. On the other hand, he did have that talent for chess where Harry didn't, though we only really see it in the first book.

For that matter, Hagrid wasn't portrayed as especially dim either; he wasn't a particularly educated character, but when the script called for wisdom of an earthier kind than Dumbledore's, he was often the one to give it. He just had a blind spot where dangerous animals were concerned, one that for mysterious plot reasons his higher-ups were happy to give him opportunities to indulge.

Well, Hermione is also the least in need of an upgrade. There are important literary reasons not to upgrade her, and insisting that the story must be exactly gender-balanced is a big constraint that limits what stories can be written.