You would think, all else being equal, that IQ would be the lowest at the center position in basketball, since there's such a premium on height there that everything else is at a discount.
For the other four in the consensus top 10 (Moses Malone, Shaq, Hakeem Olajuwon, and Patrick Ewing), I can't think of any evidence for one way or another. My vague impression from living in Houston in the 1970s was that Moses Malone wasn't considered the sharpest tool in drawer (but he could vacuum up offensive rebounds!)
As an aside, I'll toss in the story of somebody who isn't a top 10 center, but has had a long career despite playing very little basketball as a youth: Dikembe Mutumbo from the Congo. He got into Georgetown as a regular admittee to study diplomacy. Coach John Thompson just about had a heart attack when he saw this unknown 7'-2" black guy walking across campus with a stack of books in his arms. That's why Georgetown had its unwieldy Twin Towers formation with Mutumbo and Alonzo Mourning—nobody had recruited Mutumbo. He just showed up.
At other positions, you can pick out all-time greats who clearly had something going on upstairs, like Dave Bing, who became a successful steel mill owner. But, there seem to be more above average IQ types at center. At minimum they aren't less common at center, even though you'd expect height to dominate all else most at that position.
It's possible that hard-working book-smart guys do better at center than at other positions. Playing with your back to the basket is rather unnatural, and typically requires extensive coaching and drilling, whereas you can become a star guard or forward just by scrimmaging constantly.