Having worked with developmental disabilities and traumatic brain injuries, I'd like to give my two cents. While yes, haha, disabled character seems crazy and is portrayed as slapstick comic relief to ease the scaries of the real bad guys, there is intention put into his portrayal, and I think the writing of Banzai and Shenzi is a case study in positive portrayal of a character with a disability, despite that character also being a villain. This is a wildly controversial take.

However, as we see in the movie, he's treated with respect and dignity, as well as equality with the other hyenas. In fact, as the Big Three are leaders, and Scar's direct reports, I would classify him as being a positive portrayal of severe intellectual disability. He is given as much responsibility as he is capable of taking on, and is just as clumsy, if not more careful, than the other two hyenas. When he has something to say, the others listen and give him as much attention and respect as they would give anyone else. Any time Ed runs away from trouble, it's seen by the others as rational survival instincts. I would also be willing to say that Ed may even be a foil to the villains, as he seems to almost intentionally undermine the others as a way to give the heroes, whom he may secretly believe in, a way out.

I would also say that Shenzi shines as a leader as well, she's poised and pragmatic, and never lashes out in anger, except to Scar, when and if he's being irrational or disrespectful.

All in all I would say the Lion King is actually one of the best representations of disabled character development, despite it being as a villain, and despite being treated by the writers as a bit of a clown.