I know some programs that allow to import data that have a feature like
"First line contains field names".
If you have closer look at the reports then you have 2 sources:
For export it is your export-script which has all the definitions.
But as all variables in % get filled with the actual contents of the field, it is not possible to write them (the fieldnames) as the first line in a file.
Also, a report must not necessarily generate just one line for a CSV file but can produce elaborate reports in the RTF or HTML format. So it would be hard to guess what to add.
MP3tag does not interpret what a target system might do with the export result.
It may well be that you prefer the csv-export but that is not true for many other users. The design to create a decent csv-export (or any other) is in the hands of the user, MP3tag does not interfere.
For the import I would agree that it might be handy to have an option like I mentioned above. If the source program supplies such information then it would be nice to be able to tell MP3tag not to generate an error when it stumbles over such a first line but to make something of it. It might just as well be that this first line of a csv file is read and then offered as import mask for further editing.
For the export, though, I do not see much chance.