They haven't had the Shield armorer class approved yet (last I heard), but you ought to take into consideration that the normal factory spec for the M&P pistol trigger pulls involve an acceptable +/- 2 lb range.
These are "parts-is-parts" guns, and when you factor in the acceptable range & "stack" of tolerances for the various parts & assemblies, the trigger pull in any particular gun can vary within an expected weight range.
I've handled and used a fair number of M&P's since they were first introduced. Some initially had heavy, stiff & gritty triggers which smoothed out & lightened up with use ... and some started out nicely smooth and on the lighter end of the normal tolerance range.
Shooting the gun is the easiest way to "smooth out" trigger pulls.
These are "defensive" guns, not custom guns with hand-fitted triggers.
If you think something's "wrong" with your Shield 9, ask for it to be checked by the company, don't screw around with it yourself. Maybe one of the techs will take the time to try a couple of trigger bar assemblies in your gun to see if he can find one that gives a pull on the lighter end of the normal range.
(It also wouldn't hurt to check the box label and make sure you haven't received a MA-compliant gun, too.)