English simply doesn't work like that. You seem to be using "the richest person" as a sort of topic of the sentence, something Japanese does with the particle "wa," but English doesn't have this topic and comment structure. "The richest person" would have to be either a possessor as modifier, i.e. "the mind of the richest person is dull," or if there were more to the sentence, the subject, i.e. "the richest person, whose mind is dull [does something]."