You can do midi to mp3 a bit more directly. Eg. on Linux with the timidity software synth and lame installed, something like this should work:

$ timidity infile.mid -Ow -o - | lame - out.mp3

(That said, depending on the piece, I might well opt to record to Audacity from my PC as the connected Yamaha keyboard produces better playback results than anything I've managed using timidity or fluidsynth.)

From the point of view of the former folkino abc converter (and presumably mandolintab), it would be quite possible to follow the abc->midi conversion with a command line midi->mp3 conversion to give (from the users view) a direct abc->midi but I suspect processing time, cpu use and larger download files would all present problems.