Well, as far back as Mewnipendence Day in season 1, there were hints that the monster weren't as evil as everyone said and the Mewmans weren't that innocent. Which is still a story that could have been interesting, if it hadn't ended up being completely one sided.
If it had been "both our peoples made mistakes in the past, did horrible things that we regret, but it's time to move past that and live as brothers despite the danger and difficulty", that would've probably been alright. But instead it was just complaining about colonialism and wypipo.
Not to mention HOW it happened. Mewmans (which, for those who haven't seen the show, are fantasy land I-Can't-Believe-It's-Not-Humans) come to the monster (Native American) lands from a far off, forgotten country. Mewmans carve out a place for themselves, and Mewmans and monsters just kind of ignore each other for a few hundred years. Until Mewmans starting encroaching on monster territory, and the monsters understandably fought back. But it was all low level conflict.
Then ONE DAY, for NO REASON AT ALL, Queen Solaria decided that this was an affront to... something, I guess? So she creates a bunch of anti-monster killsquads, which the general citizenry was not really on board with. Then, ???, and then everybody hated monsters. Seriously, that's what happened.
Including the Magical High Commission, a group of demigods who oversee the balance of magic. Who should be above petty concepts like species or national politics. Yeah, they work closely with the Queen of Mewni, but... still.
For a show that was surprisingly subtle and nuanced at times, the whole thing was just so fucking... ham fisted.