I haven't seen any mention of Joplin support for Kroki (kroki-dot-io) in the forums.
Kroki takes any of the most used text to diagramming tools (including Mermaid and PlantUML) and spits out at least SVG for it. It has a pretty easy API. It can run locally or one can use the free online translator.
It will be extremely useful because there are 23 formats supported: The possibilities are endless
- BlockDiag
- BPMN
- Bytefield
- SeqDiag
- ActDiag
- NwDiag
- PacketDiag
- RackDiag
- C4 with PlantUML
- Ditaa
- Erd
- Excalidraw
- GraphViz
- Mermaid
- Nomnoml
- Pikchr
- PlantUML
- Structurizr
- Svgbob
- UMlet
- Vega
- Vega-Lite
- WaveDrom