I have fitted several Shadow SH 1110-UK undersaddle piezo pickups and a few unbranded generic versions to a variety of ukes from concerts to baritones. Without exception they all initially had a dominant C string when amplified. I address this either by adding one or two layers of very thin metallic tape (I think it came for use with joining multifoil insulation or something) to the bottom edge of the saddle, and then cut away the tape in the area about 6mm wide below the C string, or I file a very gentle relief into the bottom of the saddle in that location. The tape has the distinct advantage of being totally reversible. There is no discernible effect on the acoustic properties of the instruments, but the dominance of the C string is reduced to acceptable levels when amplified.
This may or may not be relevant to a Fishman - the piezos I am using have four distinct lands, one under each string.