Music notation is definitely not one-dimensional because it encodes simultaneous occurrence and simultaneous time-progression of independent pitches (e.g. a sequence of cords played legato).

And text encodes multiple bits per character. That's a meaningless distinction (or it means as much as you want it to mean in a subjective semantic debate)

I do not understand what point you are trying to make.

Yet that maps to MIDI, which is a linear sequence of events

No, MIDI is absolutely not isomorphic to sheet music.

Not necessarily true; this depends entirely on which music notation you’re talking about. Chord charts handle simultaneous pitches easily using a 1D flow of text along the timeline. Most music notations, including standard staff notation, use a layout that has a primary linear time axis, with lines that wrap, just like written text. Staff notation is different, but not that different from text. You can think of pitch as a secondary embedded dimension, but it’s certainly not necessary. You can also notate pitches and pitch combinations symbolically. The resulting audio, whether spoken or played musically, is 1-dimensional (in time) with respect to an idealized single point listener. So at some level, using anything more than one dimension is a type of feature expansion - it’s useful for notation and control, but adds new dimensions that are over-specified and unnecessary to the final result.