This would need a very delicate balancing act. Too much antimatter and the planet is destroyed, while too little and the core either does not melt fully, or not at all.
What we would hope for is the reaction releases enough energy to start the core melting, but not so rapidly that plate tectonics would be destabilized (assuming there are crustal plates on Mars). The energy release is also buried under thousands of kilometres of rock, so it will take a long time for the heat energy to reach the surface. This is probably for the best, so plumes of magma rising from the core don't reactivate the Tharsis volcanos or overwhelm the rest of your terraforming project. Suddenly discovering you are building over the equivalent of the Yellowstone super volcano is not gong to be a pleasant experience.
So the amount of antimatter will actually be rather small relative to the core, but still an industrially significant amount. Dropping it while you are setting up the worksite on the surface will probably not do the project much good...