Is Dynamic Voltage and Frequency Scaling (DVFS) enabled by default on the Nitrogen8M Mini?

Yes, DVFS is enabled by default.  

The status can be checked by cat'ing the sysfs node below:

/sys/devices/system/cpu/cpu0/cpufreq/stats/time_in_state

This will provide the time spent in each state, thus verifying that DVFS in enabled.