The migration of custom widgets is not mandatory when PPM is migrated.
If you want to use upgraded or new custom widgets of PPM 10.1 it is recommended to upgrade your MashZone NextGen version as well. Detailed information on how to upgrade your MashZone NextGen version can be found in the document Upgrading Software AG Products, chapter Migrate MashZone NextGen.
If you have upgraded your MashZone NextGen version, it is necessary to install the custom widgets again. For details see document PPM Widgets for MashZone NextGen.