Toughening can occur from doing any strenuous or frightful activity repeatedly. Psychedelics fall into this category in particular as they assault the primal part of our being responsible for fear: our egos.
I think using psychedelics for the express purpose of mental toughening may not be entirely sustainable, as this process can be egotistical in its own right. After one very tough vaporhuasca trip, I remember thinking how mentally strong I must have been to get through that ordeal. However, looking back, this thought was simply my ego recovering and trying to reconcile its prior destruction.
The higher you build a tower after an earthquake, the harder it will fall during the next one. For overcoming irrational fears, though, psychedelics can be incredibly effective. They helped me overcome many of the irrational fears I had as a result of childhood OCD, likely because such fears were based in my ego. For example, I used to be afraid of touching toilets. My thought process was, "If I touch this toilet, I will smell, get dirty, and/or get sick. I will be unclean. If this happens, people will not like me and I will have trouble living with myself."
After using psychedelics enough times, I realized firstly that such a thought process was totally irrational (and that people probably didn't like me for being so particular about toilets...). But I also realized that my desire to be "clean" was a subjective, social construct based in fear, which I no longer needed. Today I can touch toilets with ease.
So yes, they can be used for self-improvement that often is in the form of toughening your mind or emotions with regard to certain fears in your life. But I do not think that using them to make yourself a stronger person in general is that sustainable, as they may very well make you question the point of pursuing "strength" and "toughness" in the first place.
"Think for yourself and question authority." - Leary
"To step out of ideology - it hurts. It's a painful experience. You must force yourself to do it." - Žižek