Why do many light aircraft have a trim tab on one side of the elevator only? Most light GA aircraft, like this C172, have pitch trim tabs on one side of the elevator only. Why not on both? Doesn't the asymmetry add inefficiency in the form of a slight rolling moment?
A small propeller driven plane like that will have a bit of adverse yaw, rolling to the left, because of propeller effects (P-Factor, slipstream, torque, etc)

The trim tab's effect is minimal because it is very small and is close to the centerline of the plane.  Any drag it creates causes a slight roll and yaw to the right, specifically to counteract the propeller effects.

That keeps the overall effect almost un-detectable.