Exactly thisIf you're the king you should be able to go against your vassals' wants and even override a majority vote (unless you are foolish enough to have peerage/senate policy. finally i will have a reason to say "i am the senate" in this game) without having to spend silly amounts of influence, but suffer relations penalty for it depending on how tyrannical your decision was. For example if your decision had 40% support, that shouldn't cause as much discontent as picking something with 0% support. I agree that the diplomacy AI needs to be improved though, or my suggestions will become pointless as the AI lords in this game often vote as hive minds (100% or 0%) when declaring war or peace, and you'll have to suffer overriding 0% support and taking large relation penalties every time just to keep their bloodthirst in check.
Regarding diplomacy and kingdom management in general, I don't really like the idea of influence as a number that builds up over time and you can use it to force people and whole kingdoms to do what you want. Your influence over your kingdom should instead be all your relations with all other clans/lords, built up by the player's hard work and actions like doing favors/quests for them, forging ties and alliances through marriage, defending their settlements, and helping them out in battles. If people like you, they vote with you and join your armies, and if people dislike you, they vote against you and refuse to join your armies or may even decide not to join you in battle against the enemy.
However, every time you ask a favor from a lord, by asking them to join your army or vote with you on some issue, your relations should decrease a little bit, or maybe each lord should have a favor cooldown that gets shorter the friendlier you are with them, so you can ask them of more things in shorter periods of time. Becoming a "puppet master" of sorts, ruling from the shadows and wielding even greater influence than the king, your influence growing out of the support of the royal clans of the realm should be a real achievement, not something you dump influence into. The point is to make alliances and friends valuable without making them abusable. What do you think?