Toffee already knew about Meteora prior to the treaty or learnt about her around the same time it was pulled, so the reason he sabotages the treaty is not indignation about it being a joke but rather knowing that monster primacy can be lawfully restored by placing a monster queen in the Butterfly throne, but that's impossible if the queen has full control on the pusillanimous monster king and all matters of state of the monsters (through the treaty).
So he sends Rasticore to keep her safe and out of sight until he finds a way to stop Mewman magic (corrupting the well) so he can annihilate the Butterflies and claim Mewnie for monsters.
The only loose strings here to me are;
1 - How did Toffee learn about Meteora is her existence is a top state secret? Eclipsa had been imprisoned for centuries by then. Only ways I can think of are Toffee actually being centuries old and around when Eclipsa escaped with her lover: or, seeing that said monster has been missing since her wife's capture, Toffee somehow found him and learnt about their daughter from him.
2- Eclipsa knows about Toffee from Moon, but he was re-crystallized after giving Moon the Supreme Spell, so there's no way they have kept in contact all this time... or is there? I think that Eclipsa and Toffee having the same goal might be just coincidence since it revolves around someone important to them (though for different reasons), so I don't really know what to think about it.