Suggesting that people with mental health conditions do not self medicate with supplements that can be particularly harmful to those with mental health conditions is entirely reasonable.
Self-medication for quality of life is broadly all you have if you don't want to be put on the hefty big guns immediately. The alternative to the big guns is being told that there's nothing wrong with you. As such, saying "don't self-medicate" is extremely unproductive and only serves to push such practices out of view.
You are vastly exaggerating the risks of attempting to self medicate with something as safe as melatonin while underappreciating the potential benefits.