The lateral variation of the tunnel magnetoresistance (TMR) of a graphene-based vertical heterostructure is studied by spin-polarized scanning tunneling microscopy (SP-STM) using an Fe-coated probe tip. The well-defined heterostructure is obtained by the intercalation of a magnetic Fe monolayer at the graphene/Ir(1 1 1) interface. Its structure is characterized by a moiré pattern with a high corrugation. In contrast to the Fe / Ir(1 1 1) surface, graphene/Fe / Ir(1 1 1) exhibits ferromagnetic order with an out-of-plane easy magnetization axis. At the nanometer scale, our experiments reveal that the moiré pattern induces a lateral variation of the TMR, which reaches 80%. The measured TMR at valleys of the moiré pattern is higher than at hills. We interpret this modulation in terms of a different hybridization between graphene and Fe at valleys and hills due to a different graphene-Fe distance at these sites, which leads to a different transmission of spin-polarized states.