Yes, Calm Mind would be a good move for Mega Gardevoir. Calm Mind raises both your Pokemon's special attack and special defense. Because your Gardevoir is based in special attack, and Calm Mind raises your special attack, Gardevoir's special attack will be off the charts! This is especially good because Gardevoir's moves are mainly special moves, not physical moves. I don't know weather you mean special defense or just regular defense in your question, but calm mind will help for both of those stats. It would help in special defense because (again) it would boost your special defense, bringing it off the charts. It would help in defense because, even if you are good in defense, you need some special defense too, especially because Gardevoir is weak to Ghost types and Ghost type moves. Most of the Ghost type moves are special. You need some resistance to those moves, so how do you get good special defense? Calm Mind!
I personally love the move Calm Mind. My Cresselia and Keldeo know it. If you keep using Calm Mind (I've tested this) over and over again, your Pokemon will become a wall that is very hard to make faint.
So, yes, Calm Mind is a good move for any Pokemon, including Gardevoir.