There’s a very interesting “younger generation” of markmap, which says it’s based on that repo, but seems more advanced, is being updated now, and includes (apart from math equations) a gatsby plugin.
For years I tried various mindmap programs and always quit. To work well, they've got to be very good. I've found one now that allows me to type as fast as I can ideas and then just move them around easily. cut connections, reconnect. It has to be fast. It has to be much better than a simple outline. This is hard to do and I don't think it can be done with md, css, and electron. Real useful mindmapping deserves its own program.
Joplin can always import a PNG of a mindmap, or turn an html outline into a joplin document. But I don't think it can be a really good mindmap program.
I agree that the mind map needs a separate program, but this does not mean that it cannot be integrated into joplin. The following is how I use the mind map in vscode. Since it is an svg file, it can be previewed in markdown at the same time.
I think that something like markmap with joplin will already go a long way but I agree that it will not be a complete mindmap solution. Some things are already possible with mermaid but I think markmap would be simpler and faster to type.