The effect of lattice mismatch between the film and the substrate on magnetically obtained irreversibility field B_irr and upper critical field B_c2 is investigated in BaCeO₃ (BCO)-doped YBa₂Cu₃O_6+x (YBCO) thin films prepared by pulsed laser deposition on SrTiO₃ (STO), SrLaAlO₄ (SLAO), MgO, and (67%)Pb(Mg_1/3Nb_2/3)O₃-(33%)PbTiO₃ (PMN-PT) substrates. BCO has been observed to form epitaxially grown, nonelongated, and randomly distributed particles within the whole concentration range of 0-10 wt.%. Therefore, the optimal concentration of 4 wt.% BCO is selected, where the highest J_c in the high applied magnetic field range can be observed. The observed flux pinning performance is connected to the structural properties of the films, where substrate-induced uniform strain; crystallographical defects such as dislocations, vacancies, and BCO-nanodot-induced microstrain; low-angle grain boundaries; and YBCO lattice ordering, in general, have substantially different roles at different crystallographical directions.