Huh. This one of those morally gray situations where of course to us this guy seems like the baddie, but the heroes have got a mile high list of wrongdoings for why the baddie even exists?
To be fair, most of the people actually involved in the wrongdoing's are dead, aren't they?
Like, it's all well and good to say that the Pilgrims and the Unites States were the bad guy's in the whole "everything that ever happened to the Native American's", I'd definitely agree with that, and Native American's are still treated pretty terribly, but if a few years back a group of Native American's murdered Obama and Congress and took over the White House and the United States, started killing non Native American citizens within the border of the US en masse, and started hunting down Obama's surviving children as their on the run ... I'm pretty sure most people would agree that those are the bad guys in that particular situation.
Something needs to be done to start repairing the wounds of the past and fixing the civilization of the Mewni and Mewni/Monster relations, but that doesn't give Toffe a check for unlimited atrocities, and just flipping the power balance isn't going to fix anything, just continue the cycle of sins. Star is young and innocent and capable of seeing the wrongs of her own faction, that's a lot more then Toffe has going for him.
Toffe also isn't all monsters either, he's basically the heir to Seth's political faction, and Seth was notably feared and disliked even by other monsters. Seth is also, you know, arguably one of the primary actors in things being as bad as they are, refusing to participate in peace talks and establishing an organization that only seeks vengeance and violence, tainting Mewni/monster relations and all attempts to make things better to this very day.
There's no clear cut best solution here, and maybe putting Star back on the throne isn't the ideal thing to do, but I'd argue that keeping Toffe in charge is the worst result, and that Star is definitely more likely to at least try to be fair and help both sides and try to repair the rift and establish peaceful discourse then Toffee.
Edit: Star is young and innocent and hasn't actually done much wrong. Toffe's modus operandi is atrocity. I trust one of these two a lot more then the other.