You say you'd "love to roleplay a greedy, slothful coward of a ruler" but then you decry the all-too-likely scenario that such a ruler would be a maladjusted prat that nobody likes. So obviously you wouldn't actually love to be that guy.
I didn't complain that no one would like the ruler, I complained the game doesn't facilitate it. People don't like you, they conspire and they rebel, that's fine. What isn't fine is when you put down that rebellion several times and punish everyone involved. Regardless of you doing this, the AI will continue to revolt anyway until you bribe them or oust them - it has no sense of self-preservation. People don't have to like you, but there is no meter to gauge fear, the only way to stabilize your realm is to appease your vassals. There are no ways to counter opinion maluses other than to get positive maluses, even if someone would have reason to be afraid to act against you in the first place. How is it unreasonable to complain in a thread about roleplaying that you're forced into taking the same actions every game regardless of your ruler's traits or you'll be crippling your own experience?
I'm currently playing a maimed cripple with a concubine mother and a different religion than everyone else on the entire continent (excluding the wife who immigrated). Now, my stats are alright but I'm pretty sure even Mister Rogers would try to put a knife in my crippled infidel belly. And I like it fine because it's presenting a challenge.
Good for you, I can play difficult characters as well, but it has nothing to do with the point being made. My point is that when my character ends up being an arbitrary deceitful master strategist with berserker and impaler I still have to play the game by appeasing all my vassals as opposed to using repressive methods. Fighting back the rebellions and avoiding assassinations is easy, the problem is the AI will ignore the consequences of their actions and continue throwing bodies at you anyway even though they can't stop you.
Now, you say the game doesn't facilitate playing a bad guy? Well...Yeah, duh. To facilitate is to make something easier. A greedy slothful coward wouldn't have it easy - because he's greedy and slothful and a coward.
You're conflating making it enjoyable with making it easy. I don't think it should be easier - CK2 is already an extremely easy game and I certainly don't play it for the difficulty. I can easily play a tyrant and do whatever I want by abusing mercenaries and imprisoning then replacing all my vassals with new guys that don't have the tyrant malus, my complaint it isn't fun. The game doesn't facilitate that playstyle so doing so and succeeding ends up requiring gamey methods that ultimately make the whole experience really boring. EU4 faciliates using tyrannical methods such as military repression, aggressive expansion, colonial genocide and in the case of the Aztecs ritualistic sacrifice of your vassal's rulers. They don't feel annoying to use nor do they make the game easier, they simply feel like another way to play the game with its own set of pros and cons.
Getting craven in the midst of a battle is annoying, but it's annoying because traits like that are just inherently bad and don't enhance the player's fun or enjoyment in any way, it merely presents an obstacle that requires RNG to get rid of. Since CK2 is built so much around RNG the question should be how to make the random effects and traits
fun, not necessarily easier. The lunatic and possessed traits are a good example; they only serve to hinder your gameplay and make it harder, and yet most people love it when they have those traits because of the additional events and opportunities they present.