Dumb? fair enough, but dumbest of the year by far? The level of dumb is standard at worst, more intelligent than most at best. What are our comparisons? Cocaine bear? Any super hero film? Thats why the french art film comment, compared to Euro films, and if its the only hollywood film you see, fair enough, dumb as dogshit. But I'd challenge you to watch a random sample of 20 American films and argue that.
I have to say my expectations of Hollywood films are extremely low. If I make the choice to watch one, as long as there are a few interesting elements, I'm happy. Thats putting aside the guilty pleasure, so bad its good, I want to watch something brainless factor, 'that was crap, but I enjoyed it', which is a whole different thing. To take your argument to its endpoint, no character really behaves like a real person ever. Nobody ever has conversations in real life like they do in the movies.
I don't know if the characters must be developed for the film to be good necessarily holds all the time. Its definately a factor sure. But European films tend to have good character development, but can be dull as dishwater. Hollywood films often have little or no character development but can be entertaining. Oppenheimer for example, I liked it, not much character development, but you'd be hard pressed to find a better story, so IMO it was hard to stuff-up. I liked it, but it could have been done much better.
I liked LTWB, but I'll admit there were bits when I was thinking 'thats very well done', and bits where I was thinking 'come off it'. And that reflected the bits when they behaved like real people, albiet hollywoodish,, and when they didn't.
Like I said, I don't altogether disagree with you, but there has to be context and reference points. I found the film, and this discussion, interesting.