If you're familiar with GraphViz (I'm not familiar with MermaidJS, but the syntax of the graphs themselves look kinda-sorta the same) then you should be able to

If you're familiar with GraphViz (I'm not familiar with MermaidJS, but the syntax of the graphs themselves look kinda-sorta the same) then you should be able to convert a text DOT file to SVG using GraphViz then edit the SVG in LibreOffice. I wouldn't want to do a ton of edits in LibreOffice because the generated SVG isn't the cleanest. I think it should do the job if your focus is on defining nodes and edges of a graph and don't want a specific layout at the end.