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.
The reason the trim is only on one side is cost saving; one trim tab is cheaper to build and simpler (therefore cheaper) to maintain.
As for the rolling motion yes, the trim tab will cause a slight rolling motion. Very, very slight to the point you won't detect it for the following reasons:
- The trim tab is small. Compare the surface area of the trim tab to the aileron, there just isn't enough of it to make much difference
- Small lever arm: the ailerons are positioned far out on the wing so they have a longer lever arm, this decreases the force required to roll the airplane across the longitudinal axis. The trim tab is located almost on the longitudinal axis, so will have very little leverage
Besides the aforementioned cost, consider how the additional unnecessary complexity can add risk to the free movement of the elevator and rudder, should problems occur with its linkages.