Idea is simple, a command that allows a user to make their disguise terrible, when it comes to bluffing that you are terrible at disguises, someone who is good with them should be able to make a disguise bad on purpose to allow others to see through it and strengthen their view that the person is indeed terrible at disguises.
This could just be an expanded Mimic command perhaps. -mimic Disguise [number]
The number can be checked to be any number <= current disguise DC if any number higher is input it is simply set to the max.