So the princess told a bold face lie in the God's woods while standing right in front of the Heart Tree. And she put that lie on the memory of her dead mother. And she told that lie to someone who lost their mother.... and that wasn't even in the worst or craziest thing she did in just the last week.
Yeah she definitely fucking up.
And not for enough, but if the King really wanted to sow up the division with House Valerian while also hurting Damien. It seems like a better long-term plan would have been making the Sea Snake the new Hand.
That way he'd gained a direct line to a powerful ally while also robbing Damien of his partner in crime. Doing a marriage alliance gains him nothing and gives Drirkmark & Dragonstone an easy way to get up to shit without King's Landing ever knowing.
Technically, she did not lie, she did not sleep with Dameon.