Let incompetence be determined by trial an error. Wife doesn't have kids within 2 years? incompetent murder her. lol Low income per month...Steward is incompetent fire him get another. Getting caught all the time trying assassinations or too many assassinations on your life? Incompetent Spymaster get rid of him/her try another. Losing battle after battle? Levy slow to increase in size? Incompetent Marshall behead him get another. Everybody hates you, Chancellor incompetent.....remove from office try another.
It's easy to game the game or exploit it when you "know too much".....turn off stats and watch em scream in the forums. lol Players will actually try for more diplomatic marriages like the real thing I would suspect instead of going for a 20+ steward. lol
You can still let them see "traits" but not the numbers, then choices can be made by personality instead of hrmmm should I take this 17 or 18?
Create a BLIND game of it and let's see how EASY it is.