I have always heard the opposite of this. The most disturbed and unhappy are those who carry the most knowledge with them, and the ignorant are blissful. I would way rather agree with Shelley’s understanding, but I think the opposite is more applicable. He does say that they “ought” to be the happiest, not that they are. Maybe he also realized the truth but was writing what the poets and those of highest wisdom deserve rather than what they were most often given.