Here's how it ends up without mutagen:

$ mat-gui
Traceback (most recent call last):
  File "/usr/bin/mat-gui", line 18, in <module>
    from libmat import mat
  File "/usr/lib/python2.7/site-packages/libmat/mat.py", line 22, in <module>
    import strippers  # this is loaded here because we need LOGGING_LEVEL
  File "/usr/lib/python2.7/site-packages/libmat/strippers.py", line 5, in <module>
    import mutagenstripper
  File "/usr/lib/python2.7/site-packages/libmat/mutagenstripper.py", line 6, in <module>
    from mutagen.flac import FLAC
ImportError: No module named mutagen.flac
$ mat
Traceback (most recent call last):
  File "/usr/bin/mat", line 10, in <module>
    from libmat import mat
  File "/usr/lib/python2.7/site-packages/libmat/mat.py", line 22, in <module>
    import strippers  # this is loaded here because we need LOGGING_LEVEL
  File "/usr/lib/python2.7/site-packages/libmat/strippers.py", line 5, in <module>
    import mutagenstripper
  File "/usr/lib/python2.7/site-packages/libmat/mutagenstripper.py", line 6, in <module>
    from mutagen.flac import FLAC
ImportError: No module named mutagen.flac

After installing mutagen it launches:

# pacman -S mutagen
[...]
$ mat
ERROR:root:Unable to find exiftool: no images support
usage: mat [-h] [-a] [-b] [-L] [-c] [-d] [-l] [-v] [files [files ...]]
$ mat-gui # no crash

Mutagen is an optional dependency, but given that it crashes without it, it should probably be a required dependency instead:

$ pacman -Q -i mat
[...]
Optional Deps   : perl-image-exiftool: extended image support, [installed]
                  mutagen: extended audio format support [installed]