But for that we already have mermaid.js (and its precursor Graphviz/dot).

The only reason to use this instead of existing, mature ones designed for humans is if you are an AI agent.

Mermaid looks terrible.

It is only better than nothing for the purpose of showing it to people whether produced by an LLM, by a human, or by both.

Your employees may just accept the internal slop, but at some point, you have to show your charts to your customer.