Neither one of these two captains is mentally deficient. Both of them are Star Fleet Academy graduates with substantial command training and both have voracious academic interests. But its clear that one captain stands out from the other. If we scroll through the episodes and get a sense of what each captain is interested in, we see that Kirk has a love of history. He particularly admires Abraham Lincoln and American history. He obviously follows military history, both contemporary (in the case of Garth of Izars accounts) and ancient. In Star Trek VI: The Undiscovered Country, Spock gifts Kirk with ancient edition of Charles Dickens A Tale of Two Cities, noting Kirks fondness for antiques. But thats pretty much it. Kirk recognizes and respects the intellectual gifts of others, but there isnt much room in his life for academics. Picard, on the other hand is about as erudite as they come. Picard has a firm understanding and gift for archeology and xeno-anthropology. In Captains Holiday, his archeological skills come in handy, particularly in demonstrating his knowledge of other cultures. In Family, Picard returns home where we learn of his academic brilliance as a child. He also learns of a geological endeavor known as The Atlantis Project from a friend of his a supervisor of the project. It involves the creation of another subcontinent by raising a portion of the ocean floor to the surface. After Picard describes a process they used to stabilize the tectonic plates on a planet the Enterprise visited, the supervisor of the project immediately offers his friend a position, seeing how immediately Picard grasps the basic principles behind the project. It should be noted that, ostensibly, Picard is a sixteenth or seventeenth century term that is supposed to refer to someone with an enquiring mind. Picard is a natural academic, able to easily shift between various disciplines. Kirk is not.