Is it possible to generate comma-separated values (.csv) files using the Datamapper with xml-files as input?

Yes, it is possible to generate the ".csv" file by using the DataMapper, you have to define in output adf the field delimiter as "," (comma) and record delimiter as "\n" (0x0a).

If you want the file to be saved as .csv file in file system then override the ftp send method, by putting a sequential activity before the Send method which uses an MBC and that MBC should modify the file name attribute with "<name>.csv".