"True Critical" hits only exist on melee based chars. ( doubles your damage, ignores target defense etc. )
On a caster, the intelligence rate of your char and the enemy can influence the result in combat.
Low int rates on the monster you attack will of course result in higher damage you deal, as less damage is reduced by that int rates.
The same goes for mind numb, as the targets int rates are ignored then.
If you have high int rates, this of course adds to your total damage, speaking of increasing the minimum damage and the maximum damage further to the top, so in total your average damage is bigger.
Basically, the choice of charm is personal preference, the average damage will be about the same in the end,
but in single combats you can be more or less lucky with a lightcharm for instance.
to get an overview on the charms:
lightning charm: very high maximum damage, very low minimum damage. huge difference between this points. for lucky ones or with good gear, 0-9 damage on level 1
wind charm: very high maximum damage, very low minimum damage. huge difference between this points. for lucky ones or with good gear, 1-8 damage on level 1
fire charm: higher maximum damage, lower minimum damage. big difference between this points. but more controllable damage, 2-7 damage on level 1
earth charm: high maximum damage, medium minimum damage. medium difference between this points. even more controllable damage, 3-6 damage on level 1
ice charm: medium maximum damage, high minimum damage. small difference between this points. very controllable damage, 4-5 damage on level 1
I personally like the consistent damage of ice charms, but you are right, the damage of a lightning charm in connection with mind numb can lead to very high maximum results.
But you always have the risk of hitting just for a very low minimum damage.
You can find a quite helpful comparison over here:
http://forums.d2jsp.org/index.php?showtopic=20806226&f=272