To what degree is incompetence is allowed? - It's allowed up to the point you're either causing more harm in the round than not, or you're part of a command role. In the case of a mute SL, (check the rulings above about SLs) this is not allowed and mods should be taking as SL is a vital role in the round. If someone wants to not bother with RP, we got roles like PFC for that. In the case of a Doctor, a Mentor will help them out to the best of their abilites but unless the doctor is killing people with his operations, taking a long time to heal you up isn't something we punish for.
this is an official ruling from Emerald. Unless you don't include queen as "command role" then this should apply to queen as well. Queen straight ruins rounds with incompetence. So can we now report queens for being incompetent?