When it is necessay to download the graphs as svg (very useful) or png, the graphics can be edited anywhere and then be pastes on Mermaid Live editor.
For editing more comfortably, Virtual Studio Code can be used with some extensions:
For my daily work I use Notable's built-in editor. Notable is a Markdown notes organizer. Markdown is «a lightweight markup language that you can use to add formatting elements to plaintext text documents». Notes taken in Markdown can be inpedepently opened by different programs: you can create them with any app and open them anywhere with the same or different apps.
Right now I'm creating a DB in Filemaker for visually creating graphs: the user just fills in the nodes and a script creates the code around.
Russell Watson has an introduction about how to integrate Mermaid into Filemaker and some demos.