You're pushing a rock up hill--many markdown renderers natively support mermaid, plantUML, etc. using fenced code blocks.

If you explicitly want source code rendering don't add the mermaid tag to the block. There could be an argument that now you don't get syntax highlighting, but remember syntax highlighting itself is not part of any markdown spec and it differs wildly between implementations--there is no common grammar or spec for it even.

In all cases these are still valid markdown files and will pass through markdown processing, rendering, etc. systems without breaking them.