MIDI sounds bad because while the notes being played respect a mathematical structure, the MIDI instruments spectrums do not integrate properly into said structure.
Yes of course, mathematics is used in the design of the instrument but it's not really a "mathematical" / precise device in the same way a calculator is, nor does a good musician conform to mathematics only when expressing themselves using the instrument.
In traditional musics, people use stones which when hit will make a sound that might conform to some frequency, but I wouldn't think the stone is "mathematical"?
I completely understand math is involved in Western and Eastern musical theory, but a saxophone being "mathematical" ? My trumpets and saxophones will only play the correct tones if I work with it to do so.
Disclaimer: been playing sax, trumpet since I was 6.
This is why the "timbre" of a piano is different from the timbre of a sax, and why the same C note might fit in a song if it's played by a piano, but not in the same song if it's played by a sax.
If you take a higher level view, the mathematical structure of the notes (the chords, ...) needs to fit the mathematical structure of the harmonics of the instruments (the timbres) to get a pleasing sound. Put another way, the timbres of some pairings of instruments might clash on a particular sequence of notes, so you need to search for a different instrument pairing which is harmonious on that sequence.
And then you have synthesizers, which are very mathematical in the way the timbre is generated (oscillators, filters, ...), but this allows you to exactly fine tune the timbre so it "fits"
We can use mathematics to describe the theory behind music but the theory isn't the music itself, nor are the instruments, "mathematical".
When a great performer plays live, they're not generally perfectly reproducing what's written in the musical notation either.
Probably not a straightforward thing to argue about, as I said, I think it's mostly a matter of viewpoints.
I have no opinion either way, but this conflates mathematical with automated, doesn’t it?
