Hello,
Generally, you would have to create a SQL dump file when you are trying to import data into Cloud SQL. This process consists of creating a storage bucket and uploading the dump file in the bucket before import into Cloud SQL. For the purpose of clarity, would you be able to confirm if the process you are using to import the data fulfills the requirements found
here? This link also includes steps to import data into a Cloud SQL instance.
Also, in regards to your specific question, my research indicates that .tar files are normally used by Linux/ Unix systems. These files are only meant for storing data and not compressing it. This could be the reason why you are encountering the error message you are referring to. I was able to find out that the the supported compressed file format for Cloud SQL is .GZ. You may compress your .tar file in order to make it into the supported file format for your import action. For further information regarding .tar files, you may consult this third party documentation
here.
I hope this helps!