English Translation
He’s so dull!
According to the Clozemaster translation, “bouché” does not mean dull. It means dense or reflects someone not capable of understanding something.
English Translation
He’s so dull!
Dull can also mean not capable of understanding.
Merriam-Webster Dull 7 : mentally slow : stupid
I suppose it depends on context, but I read the English sentence as meaning “He’s so boring.”