Stupidity: So when Belle finds out that her father is being hauled away to the insane asylum, she is in such a rush to get that she doesn't even change out of the massive yellow dress that would be really uncomfortable to ride a horse in, probably because she is in such a hurry. But earlier in the movie, the Beast shows her his magic book that allows him to teleport anywhere in the world, so why doesn't she just use that?
Suggested correction: As you've just stated, she is in a hurry. She probably didn't think about using the book.
Stupidity: When Jasmine finds out that Prince Ali is actually the boy she met in the marketplace, she seems to completely forget about Jafar telling her he had been executed. She never asks why he's alive, nor does she even bother to confront Jafar about his treachery.
Suggested correction: The fact Aladdin was alive only told her Jafar lied about it. When she is brought back by Aladdin to the palace it is still night, not long after that she is confronted by her father and Jafar who has him under a spell. She hardly had time to ask about the treachery, probably thinking to do it in the morning. It is there and then Jafar is exposed by Aladdin. She probably wondered about it, but didn't figure out the implications yet.
Stupidity: After all the singing and dancing at the Tent Revival, Lt Elizondo yells at her fellow officers "It's a Trick, they've brainwashed the Captain, get 'em." Considering her and everyone else just witnessed the Captain float into the sky surrounded by fireworks and miraculously change into a black suit her "brainwashing" assumption is truly stupid.
Stupidity: For claiming to know how to deal with Music Ebo doesn't do a good job. He wrestles her to the ground to calm her down which is very dangerous for both the patient and the caregiver. It's only recommended as a very last resort and in both scenes with this neither one required it to be used.
Stupidity: After leaving the church in the painted Studebaker, the duo have Max and Hopper right behind them. Therefore, there's no way the (conveniently) colored car could possibly fool Max and Hopper.
Suggested correction: That was the whole point. It was supposed to be funny.
Stupidity: If the fairies knew that Maleficent was going to try and kill Rose on her 16th birthday, why, oh why, do they take her back to the castle on that day? Why take her back to where Maleficent was expecting her? Why not wait until after her birthday?
Suggested correction: We see Ariel sign in English, but most likely they have an entirely different alphabet and language from our own.