I think there is a line, when playing to the intelligence of a creature, which stepping across means choosing smart & not as much fun over smart & fun, and as a DM I choose never to step across that line under any circumstances.
Usually, this means the difference between me playing an enemy that is using available strategy in order to achieve victory, and me playing against my own players.
It all comes down to seeing those choices that are antagonistic (to the players) and being able to choose another thing that is also smart of the NPC to do. For example, when it comes to an unconscious but not dead party member: It is smart to intentionally strike the downed character so that they can't rejoin the fight; It is also smart to identify the still standing character that is capable of returning the downed character to the fight and attempting to prevent them from doing so - not as smart as killing off the downed character, sure, but still plenty smart and also more fun because it heightens the tension by leaving the uncertainty (the question: will the character die, or will the character rejoin the fight?) uncertain for a longer period of time, while giving the players something they can engage with and overcome in the moment (rather than later via returning from the dead).
Also, having known a significant number of people with "genius" IQ, I know it is extremely common that such people have complete blind spots for certain kinds of things, and actually make what others might call "dumb mistakes" far more often than people with IQ in the "norm" do - if for no other reason that because they were so busy thinking of all the complex this and thats of a thing that the obvious either slipped their mind or was thought of and refused because "...it can't be that obvious, can it?"