No, it is not necessary. You need not re-run the complete migration if few user mappings were missed during the job / task creation.
For Private Chat Migration, the User mappings that were missed during the task creation will be listed in the TASKS -> EDIT option. You can add the missed mappings from here.
For all other migrations, we can add the mappings in Mapping -> USERS MAPPING. Then run the task again using the option “Retry”.