No, only successfully migrated files are counted towards credit usage. The size of these successful migrations is deducted from the Credits Assigned, and the Credits Left value is updated accordingly. Cancelled or failed file migrations do not consume any credits.
PS: View Mass File Migrate Salesforce file migration app on Salesforce AppExchange:
https://appexchange.salesforce.com/appxListingDetail?listingId=e9874a66-2001-4e3d-8a29-3a2ed7f584c4