In this paper, we developed a multiphysics mesoscale model of carbon fiber oxidation in the upper most layer of a phenolic impregnated carbon ablator thermal protection system. The presented model uses the phase-field method to capture the reduction of carbon fibers due to an oxidation reaction between carbon and oxygen to form carbon monoxide. The model is fully coupled with heat transport in the system, including the heat absorbed by the endothermic oxidation reaction. We implemented the model in the new Macaw application, created using the Multiphysics Object-Oriented Simulation Environment (MOOSE). The model was verified against an analytical solution of surface reactions. A sensitivity analysis revealed that the parameters that most impact the oxidation time are those related to the reaction rate. The model was demonstrated using 2D and 3D simulations of the oxidation of multiple fibers, illustrating the impact of fiber diameter, temperature, and gas flow on the fiber oxidation.