Duskull, according to the wiki, should spawn in mesa biomes. When checking my server using the "/wiki duskull biome" command with pixelmon extras, the response I got was "Duskull spawns in these biomes: none". I also have not seen any duskull, so I assume it is an issue with pixelmon and not the pixelmon extras command. The server is running Version 1.12.2-7.0.5.

Thanks for the report. This is currently an issue with Pixelmon Extras -- it does not handle files with multiple sets of conditions properly. Duskull should spawn in the Nether, or in Mesa Plateau F biomes at night. It's a common spawn in both, although finding the specific Mesa biome required may be difficult as it's a pretty rare variant.