I am playing on 1.16.5 with pixelmon 9.1.11 and even with a datapack with pixelmon npcs in the no_dirt_spawn.json and no_spawn.json they still spawn.

You can figure out how to block them? I'm currently making an modpack for me and my friends, and I cant deny the mobs from born in chaos to spawn in dreadful dirt

If the correct data pack doesn't work then no, I ended up giving up and using spawners for specific mobs instead.