Is your feature request related to a problem? Please describe.
I loaded a schematic for a zombie pigman farm, and it included the pigmans.

Describe the solution you'd like
Add a setting to control rendering of mobs/entities, maybe it would be useful to be able to do this based on entity type blocks vs. mobs.

Describe alternatives you've considered
One could edit the schematic and remove all mobs.

You can remove/disable entities entirely from the placement settings.