Currently, JCMA doesn't support copying data for "Issue picker" custom field as described in  

https://support.atlassian.com/migration/docs/what-gets-migrated-with-the-jira-cloud-migration-assistant/

Users have to manually recreate this field in Jira cloud instance and use Import data from CSV approach to copy data.

This is manual and time consuming process.

Provide an option to copy data for "Issue picker custom field with JCMA .