When there is a hierarchy of process instances (using call activities), then an incident is created at each level of the call activity. The ProcessDefinitionStatisticsQuery provides already the option to include incidents into the result (see #includeIncidents()):

managementService
  .createProcessDefinitionStatisticsQuery()
  .includeIncidents()
  .list();

The resulting statistics includes per process definition the existing incidents.

AT:

• there exists another query option like #includeRootIncidents()
• when this query option is used, then only root incidents are included in the resulting statistics
• query option is exposed in REST API

Hint:
An incident is a root incident, when the ID of the incident is equal to the "root cause incident id".