From what I can tell Finn and Jake only pay taxes to the Candy Kingdom not because she owns the Grasslands but rather because Finn and Jake seem to see themselves as her citizens.
The Grasslands seem more like a designated principality of city-states with a hundred smaller Kingdoms like the goblin kingdom the cube guys the house villagers the fluffies and mushrooms and all sorts of humanoid villagers running around and I highly doubt any of them see themselves as citizens of the Candy Kingdom.
I don't think Finn and Jake care about getting taxed, but I don't think PB has any legal ground to stand on here and she's only getting away with it because those two like her and won't refuse; if anything they'd be happy to help.
I take a moral objection to this because I am not nearly as altruistic as Finn or Jake and I disagree fundamentally with the idea that anybody should be allowed access to a person's funds when they aren't home.
I also see what you mean with that real life analogy but this takes place in a dystopic future and frankly real medieval societies shouldn't be used as context here. Ooo seems to operate on a Greek City-State formula where a kingdom consists solely of its capital and immediate surrounding territories. Neither do they seem to have standing armies nor any real need to defend them, likely due to the influence of heroes defending them from all threats.
If anything Finn and Jake should be viewed as an international organization as they fix problems from a state to global scale and therefore immune to paying taxes. If anything the princesses should be paying them for services rendered.