You're not likely to ever hit the crit cap.
Hit + Crit + Miss = 100%.

As you increase your hit percentage, your miss percentage decreases.
And as you increase your crit percentage, your hit percentage decreases.
It is theoretically possible to get to a point where you have no hit left so additional crit is impossible, without first increasing hit (decreasing miss chance).
Not likely to get there.

As a caster, against a raid boss, with no hit gear, you have say 17% miss.
Which means there is 83% chance of a hit or crit.
If you were 83% crit chance, anything beyond this gets you nothing.
At least until you increase your hit, which lowers your miss, and gives you space for more crit to be helpful.

I believe the worst to-hit is dual wielding.
And against a raid boss, rather then an opposing player.