Not a bad idea, but to prevent too much determinism and the tendency the player may have to kill its own ruler, maybe just a few handicaps/incentives could be added. Something like that your "coward, weak-willed and amiable King" would require a higher warscore to demand compensation on an equal than a "vengeful, proud, stubborn, reckless King" and such. I suppose this dynamic could be coded in the traits.txt file like: "weak_willed = { demand_compensation = 2 } meaning the warscore needed to ask for that in a peace settlement for a ruler with that trait would multiply for 2, so it'd make it a lot more difficult to happen and would help the AI to choose other more feasible and perhaps peaceful peace options. Just saying.
I prefer this. I want traits to have consequences for others, not for the human player making decisions. If you handcuff the human player, the game becomes too deterministic and it will play itself. I.e. it will become boring.