I find ASCII diagrams to be the opposite of Markdown. Markdown isn't perfect and so that is where extensions come into play, but ASCII diagrams are more like man pages. I think ideally a diagram markup software would use YAML or some other similar convention to show hierarchy while still be readable. Typing and editing ASCII diagrams by hand is simply not convenient.
On YAML-like diagrams, I think you are looking for mermaid [1]. I wrote down here [2] how typograms compare to mermaid [2] in terms of trade-offs.
[1] https://github.com/mermaid-js/mermaid [2] https://code.sgo.to/typograms/#related