Okay, I've got it; I can now get the process parameter spread by including

critical.spice

followed by

montecarlo.spice

, in place of using

.lib sky130.lib.spice

. The top of my testbench file looks like:

.param mc_mm_switch=0
.param mc_pr_switch=1
.option SEEDINFO
.option SEED=random
.include /usr/share/pdk/sky130A/libs.tech/ngspice/parameters/critical.spice
.include /usr/share/pdk/sky130A/libs.tech/ngspice/parameters/montecarlo.spice

I think it would be simpler for most people if I just added a block called

mc

to

sky130.lib.spice

.