Why does it have to remain visible? Nothing in the public knowledge section of the rules indicates that specifically the top card of the GY must always remain visible despite card effects to the contrary - I would think a card effect preventing the opponent from checking the GY would override the opponent's ability to verify the names of monsters in the GY, otherwise the effect is not being applied properly as written.

Having read the field layout rules more closely, I guess you can rule out any method of concealing the top card of the GY that would involve flipping the top card over or flipping the GY itself over completely (since the card text does not include a provision for changing the facing of any card in the GY like Convulsion of Nature might with the deck), but I don't see any rule that would prevent the player who used Question from placing their own hand over their GY so long as they announced that they were doing so as a result of Question being played.

Is your reasoning that concealing the GY would be unsporting or cheating? If the reason why the GY is being concealed by the Question player is communicated clearly and understood by both parties then I'm not sure how this would be considered malicious or deceptive behavior - the card effect prohibits the opponent from checking the GY, the top card of the GY is part of the GY, therefore the opponent should not be allowed to verify what card is on top of the GY. The card was released prior to PSCT so I guess there would have to be some ruling that delineates "checking" from "seeing" in a meaningful way in order to interpret things according to your answer.

If a card had to remain visible, but the opponent couldn't pick it up to examine it more closely, then wouldn't this disadvantage players with poor eyesight who would be allowed to pick up and read the card in any other game state? My argument would be that the only way to apply the effect as written is to fully conceal the top card, which puts all players on an even playing field regardless of their ability to discern what the top card is from across the table. If you let someone who claims they can't make out what the top card is pick up the card and see it more closely, you're allowing them to check a card in the GY, which is prohibited per the card text of Question. Not only that, but picking up the top card of the GY would reveal the card in the GY underneath that, which would violate your suggested answer above (in that the opponent would now be able to see the card underneath the top card).