A good passive mixer (for instance, a balanced mixer) is well matched at its RF port over a specified bandwidth. It may be difficult to achieve the same degree of matching at its LO port.
With a nominal LO pump power, the RF port is matched, so the mixer behaves as a good load to the LNA connected to it.
For instance, Spacek Labs. Inc. makes a series of waveguide mixers with a good match (RL>10 dB)over waveguide nominal bandwidth. Also available are ultra-wideband balanced mixers using WRD-180 RF input waveguide; those operate over 18 to 40 GHz.