@Kuinkwin People this stupid do exist. The problem though is the context and setting. The idea that people this stupid would all be in the positions they are in without the kingdom burning to the ground in a week is pretty laughable. I mean they aren't even able to keep tabs on their neighbor well enough to know the kingdom has been conquered.
The other issue is just probability. Like it is possible for me to win the lottery 20 times in a row? Sure. Would people rightfully call bullshit if it happened? Yes. Meaning just because you can argue something CAN happen doesn't mean it should happen in any reasonable scenario.
And not only are these people as stupid as humans can possibly get. The idea that despite being this stupid they would all have unreasonable bloodlust against the heroine is stupid. Even the younger prince is like "LOL YES BLOOD FOR THE BLOOD GOD!" like WTF why is that kid so damn blood thirsty? Why is the king making excuses to kill this girl that is technically one of his In-laws? Even if her own family mysteriously doesn't give a damn about her shouldn't he have some thoughts against killing his original daughter in law? But here he seems damn eager to behead her himself for no explained reason than "He magician dude told him she was a witch" and he just apparently believes it without question.
TL
R-Characters can be stupid but the plot itself shouldn't be. The plot should still flow in a reasonable way and not rely on pure stupidity to even function.