interesting question, I don't see the connection.
I will go along with all truth is belief.
Any lie you tell yourself long enough, you will believe it and it will be your truth. You will even be able to pass a lie detector test about it.
What is your definition of mental illness?
If you accept clinical diagnnoses as legitimately identifying abberant and abnormal behavior resulting from brain dysfunction, then you believe in the existence of mental illness, by definition.
The only way conspiracy theory could come into the discussion is if you simply don't believe in the existence of brain dysfunctions that result in abberant or abnormal behavior.
I would submit that there is overwhelming evidence that such behavior exists, and I am willing to accept brain dysfunction as the cause of most of it.
Who would you be pointing the finger at as the co-conspirators?