I agree that it is no ideal, at least for my tastes, but I think what you are stating is an exaggeration. If you are using reference points like someone that knows little to nothing about technology, then just about anything different that what they are used is probably as readable as C++. For example, trying to read HTML would be the same to my grandma as trying to read C++.
Readable by humans is relative. Mermaid does not make it unreadable to some humans. Again, you are free to create an alternative that makes it more readable. Some could say that adding tables to markdown makes it unreadable. There are lot of extensions for markdown that adds features, but you have to know what they are to read them and use them. Grandma doesn't have to use Mermaid if she doesn't want to.
