As is documented in the LAMMPS manual, for almost all kinds of systems, especially dense systems, the MPI parallelization in LAMMPS is more efficient than the OpenMP threading. In addition, the OpenMP implementation in LAMMPS is tuned to be particularly efficient for smaller numbers of threads (usually no more than 4, sometimes 8, rarely more like on (now defunct) IBM BlueGene supercomputers).
Please see the responses to How to reduce kspace timing% for more discussions on how to determine and optimize performance in LAMMPS in addition to the manual: 7. Accelerate performance — LAMMPS documentation