Regardless, yeah. It's clearly NOT diminishing, as Waliel said:
If your crit is such that you have a 5% crit rate base, you deal 150% damage on a crit. Thus, adjusted base damage is:
100 x (.05*1.5 + .95*1) = 102.5
Increasing it to 6% (and thus Crit damage to 151%):
100 x (0.06*1.51 + .94*1) = 103.06
Difference: 103.06 - 102.5 = 0.56 -> 0.56/102.5 = 0.546% increase in damage
And similar calculations for 20% (165%) -> 21% (166%):
100 x (0.20*1.65 + .80*1) = 113
100 x (0.21*1.66 + .79*1) = 113.86
Difference: 113.86 - 113 = 0.86 -> 0.86/113 = 0.761% increase
However, if you get rid of the crit rate increasing crit damage, the crit mod is always 1.5. If we plug this into the above formulas, we can actually solve everything. Consider x to be the current crit rate as a percentage. Below, I solve to find the net change in potency that is gotten when increasing the critical hit rate by 1%:
net change = [ 100 x (((x+1)/100)*1.5 + ((1-(1+x))/100)) ] - [ 100 x ((x/100)*1.5 + ((1-x)/100)) ] -> simplify:
net change = [ 1.5(x+1) - x ] - [ 1.5x + 1 - x ]
net change = [ .5x + 1.5 ] - [ .5x + 1 ]
net change = .5x - .5x + 1.5 - 1 = 0.5
Considering the fact that, as your crit rate increases, the expected potency of each hit, adjusted for crit, increases as well, even with constant crit damage percentage, the fact that you have a constant rate of change (rather than variable) leads to a pretty clear picture as to why what Waliel said is completely accurate.