A frac plug mandrel assembly has an inner core that permits flow that is surrounded by a thin wall tube that distributes compressive loading, such as that applied by the set sealing element to the core. The core is a cylindrically shaped insert for inside the tube and allows flow until a ball or plug is landed on a seat to close off the flow through the core. The core can have a star pattern with a series of radially extending segments from a solid hub or a cylindrical shape of a honeycomb or other porous structure that has the requisite strength to resist collapse from compressive loading of the set sealing element while still allowing sufficient flow area for fluid displacement.