Is acrylic fabric on garden bench cushions waterproof?

Yes, but only up to a point. Acrylic fabric is normally waterproof to water being splashed onto it, but like most 'waterproof' fabric, there will be a "capilliary" action that leads to water soaking through if it is left on a wet surface. Basically, the water gets drawn up into the fabric by a wicking process (this is why you can sometimes get soaked in a downpour if you are wearing a single layer anorak or raincoat, rather than a dual layer coat which has a lining).

You can get around this by either using 2 layers of waterproof fabric, or by purchasing a specialist fabric designed for that use (Just google "waterproof garden fabric" or similar to find such fabrics).