What do you weigh?
20psi is pretty low for a typical 2.25-2.5 MTB tire, but if you're not getting any deformation, squirming, or pinch flats maybe it's ok for you?
For comparison, I'm 210#, and typically run 30 in a 2.4" tire. With a 2.6 I'll go down to 28, 3.0", maybe 25ish psi. 4-5" fat tires 7-10psi.
For road I max them out.