If you only have multi-timbral instances of Kontakt in your project and no other tracks, there can be an advantage to using the multi-processor option in Kontakt. However, once you starting mixing those Kontakt tracks with other instruments or audio tracks with cpu-heavy plugins, the multi-processor option in Kontakt tends to conflict with the efforts Cubase makes to spread your cpu load across multiple cpu cores.
There are exceptions and you should run some tests to to confirm what works best for you, but the general rule is: leave the handling of processor assignment up to the DAW and don’t try to do it in the instrument. Similarly, it usually works best to run instruments mono-timbrally instead of multi-timbrally. That makes it easier for Cubase to efficiently spread the cpu load across the cores.