There would be no reason to topple a human king and put another human in his place. From the demon king's perspective, that's just switching one random human for another.
Making humans hate the "star maiden" on the other hand would have significance. The maiden is supposed to be a special human born to solve a crisis for all humanity. Like, say, a campaign of extermination by the demon king. Having her hated by the other humans would be an effective way to get rid of her without risking a direct confrontation. (Which is exactly what would have happened if the villainess had fulfilled her original role of summoning him to destroy humankind.)
You see, objectively this scenario is about as plausible as the one where the maiden was the one manipulating people.
Try to ignore what you know of the story from Remilia's POV, and try to place yourself in the role of someone who knows his own emotions are not to be trusted, because they have been manipulated at least once.
... and possible twice. Because those two ideas are not incompatible. There is the possibility that the star maiden did manipulate people and now the demon king is doing it too.
this can be the case if you do not consider what will happend in the next chapter and what Remilia found with the mirror
and considering that the star maiden is already hated for what she did in the high society, i think that the demon king do not even need to brainwash people to hate her, you can see people's reaction of her joining the party even before drinking the wine, and after drinking it their reaction on her saying that she should marry the demon king
even if you are brainwashed by Pina or Demon king, wouldnt be your first thought: is this woman ok with her head? saying it, in front of her fiance the prince, her king, the queen, the demon king, also the rest of high society, it's like asking to be executed on the spot if she didnt have some protection as star maiden and the prince's fiance
and considering the propaganda that remilia did to change people's view on the demons, brainwash via magic is really uneccessary, capitalism does it even better and you will be glad to join their side: buy my lilin wine, and for the first 300 people, i'll throw in also an anti-curse brew for all type of aliments, you can get drunk while you detoxify from your other aches