You could start out that way and get pretty far, but eventually the language will get complicated enough that you'll want something more like LaTex than like JSON if you want it to be readable.

Musical notation is often deliberately ambiguous to allow for interpretation. Tempo is often approximate. Durations are often approximate or implied (such as swing time). Volume is very approximate. And then there are various kinds of hinting. The hints need to be in the source code because nobody wants to specify them with more precision.

But that said, I'd be perfectly happy with something basic that runs in a browser.