Well, it won't inherently be low-quality, but if you want to fit the map under a reasonable limit (4mb, of course, at the max), you'll have to cut corners somewhere.
I know MIDI is used for a couple minor things in Warcraft 3, like environmental sounds or somesuch, but I'd heard that some people had experimented with abusing that and found that it really screwed up the sound. I wouldn't be surprised if the Map Utilities just converted the .mid to .mp3.