In order to automatically mail-enable Public Folders, the migrating user must be an Exchange Administrator.
The Exchange Management Shell commands that are used are only available to an Exchange Administrator, so if you are using a user that is not an administrator when performing a Public Folder migration, you will get an error stating: "The account being used is not an administrator. Please elevate the user to an administrator in order to mail-enable public folders."
This error will not cause your migration to fail, but the folder will not be mail-enabled.