Efficient spin injection from epitaxial ferrimagnetic NiFe2O4 thin films into a Pd layer is demonstrated via spin Seebeck effect measurements in the longitudinal geometry. The NiFe2O4 films (60 nm to 1 μm) are grown by pulsed-laser deposition on isostructural spinel MgAl2O4, MgGa2O4, and CoGa2O4 substrates with lattice mismatch varying between 3.2 and 0.2%. For the thinner films (≤330 nm), an increase in the spin Seebeck voltage is observed with decreasing lattice mismatch, which correlates well with a decrease in the Gilbert damping parameter as determined from ferromagnetic resonance measurements. High-resolution transmission electron microscopy studies indicate substantial decrease of antiphase boundary and interface defects that cause strain relaxation, i.e., misfit dislocations, in the films with decreasing lattice mismatch. This highlights the importance of reducing structural defects in spinel ferrites for efficient spin injection. It is further shown that angle-dependent spin Seebeck effect measurements provide a qualitative method to probe for in-plane magnetic anisotropies present in the films. © 2020 American Physical Society.