If you use copy and paste in macOS, it's up to the target app to decide which contents from the pasteboard take priority. Mp3tag provides a variety of representations of the files you're copying:
- file promises for copying in, e.g., Finder or other file-management related apps
- file URLs for apps that take URLs
- a textual representation that is created using a format string that is configurable via Preferences → Export → Copy Format
Regarding Numbers: I don't know how to make it paste into separate columns (I'm not a Numbers user). A quick test showed that copy-/pasting tab-delimited data from a plain text file also does not separate it into different columns. Do you know how to make this work?
However, pasting to a CSV file or using File → Export... as indicated by @MotleyG works (you can also drag&drop the CSV file).