I have been using Monodraw for a long time: https://monodraw.helftone.com/
It's a macOS app and I've found it great. However if given an ASCII diagram, you cannot edit it with the same ease as creating a new one (e.g. reflowing text or resizing boxes).
I really like the idea of having the mermaid source and the ASCII diagram together, so you could use the source to change the diagram if needed. But I feel that would feel cluttered to have both in a plain text file or comment, where ASCII diagrams shine.