After you reach the "bottom" of a normal keypress you keep pressing through to activate aftertouch. Where this bottom is (and aftertouch begins), where aftertouch ends (maxes out at 127), and how it acts between these two points are fixed in hardware. There's no adjustment.
However, when you are using aftertouch to change values of a destination parameter - this is where you would apply some "curve" using aftertouch as an input and the result of your curve as the output. You could make everything between aftertouch=1 to aftertouch=120 (all except the last values) output a 0. Then values 121-127 output some larger values. This would mean that not until you "bottom out" aftertouch would it do anything. Likewise, you could set aftertouch=1 to aftertouch=32 output rapidly increasing values to from 0-127 then aftertouch>32 keep 127. This would set a very "light touch" for aftertouch.
If you want aftertouch to overall be less sensitive - meaning even with trying to use modulation curves you're not able to do what you want - there's not a way to do this. How the aftertouch controller acts is fixed.
It's how you apply the outputs of aftertouch where you can map values differently and create your own aftertouch curves.
Remember this is not polyphonic - so pressing down multiple keys, aftertouch is going to be the output of the greatest "through bottom-out" key pressed. So if your right hand is lightly touching and wouldn't be creating any aftertouch events, but you have your thumb mashed down on the lowest key - the aftertouch controller may be outputting a steady 127 until you let go off the thumb.