How Is Witchcraft Linked To Mental Illness?

Its not officially.
In fact if you are talking about witchcraft as in the the practice of wiccan rituals etc, IMO there is no link what-so-ever.
True wiccan crafts, spells, rituals, whatever you call them are very healing. Such a beautiful, amazing gentle religion  that values things that promote wellness, such as balance, acceptance, self love, natural co-existence.

Whereas cartoon garbage sadly popular misinterpretations of witchcraft may have ill effects on mental health. With the promise of spells that return lovers that have moved on, or control the wills and mind of others.. Eek, we are talking about the encouragement of obsessive focus, unhealthy desires for e.g. To be in a relationship that doesnt work, and often, being vulnerable to the scammers pretending to be witches while parting you with your money and leaving you broke, confused and emotionally depleted.

True witchcraft involves finding peace in self and nature, loving and valuing self, taking control of oneself and being one with the universe. Real witchcraft promote mental wellness.
That other stuff passing itself off as witchcraft supported by story books and the myths shared by ignorant opens the door to mental instability.