Not true. Every trainer I've seen has enough fuselage height to mount the tank properly. Sticks are notorious for needing a side mount, but generally not trainers.

That said, you probably should check your tank height. The centerline of the tank should be even with the spraybar of the carburetor. Up to 1/2 inch difference up or down is acceptable, but more than that will cause fuel mix problems if you aren't running a pump.