Mostly yes. Self medicating, mentally ill persons tend seek like minded company.
Your statement (if true) proves only that mentally ill drug users mostly share drugs ... it does nothing to prove your claim that most people who seek to share drugs are mentally ill. Do you have any proof for that claim?
Yes. The increase of drug abuse and its attendant rise in crimes of property, violence and murder since the passage of the Community Mental Health Centers Act of 1983 and the end of institutionalizing the mentally ill. This act was the beginning of the phenomenon known as the "60's" and all its attendant evils since then. Especially drugs and drug violence and smuggling. The Heroin poppies that now grow in the fields of Afghanistan, can be traced to the policy of letting the mentally ill to live in the streets.
Look, I know you enjoy your pot, and as long as you don't encourage others who do not smoke now to smoke with you, I don't personally care what you do. But the fact remains that many people who are now in prison or homeless on the street is due to the lack of treatment of their mental illness and their introduction to, and the use of, illegal drugs.