Would Nature 20 be appropriate for the mighty Cthulhu?
His Might of 8 already means that players generally can’t engage with him. Throwing Nature 20 on there is just a way of saying “Even if you are trying to riddle or flee, you lose, always.” If that’s your feeling about Cthulhu, I’d just not stat him out and have him be a source of twists instead of an enemy so as to not waste time rolling for Disposition.