The project itself is cool, but I'm doubtful on the philosophical side.
Mermaid aspires to be the Markdown of diagrams, so being editable and consumable via plaintext is paramount. If I have to use a visual editor, why not fetch for more powerful tools and be liberated from the limiting format?
This is akin to a WYSIWYG Markdown editor (which to be fair has a niche market). I would ask why not just use Google Doc or MS Word at that point?