Nobles can’t use Maid Spy on the king, since that would give them too much free info and would make whisper games even stronger for BD. That makes sense.
However, I suggest giving Aristocrats and Apostles the ability to spy on the king. That would add another reason to be cautious about whispers. I don’t feel it’d be overpowered - it’d come at a serious risk. Since Nobles still can’t spy on the king, you could get caught if someone asks you who you’re spying (you won’t be able to report whispers from / to them, and obviously you can’t admit you spied on the king.)
Naturally, they could still spy on anyone else, too, so it’d be up to them if they want to take that risk on any given night (you could always claim to have spied on whoever died, even if that might make the court suspicious.)
EDIT: A more dramatic change would be to just give them the ability to hear all whispers.