For graphviz, the "problem" with it is that it truly is general purpose and trying to make a general purpose syntax for diagrams is difficult. Mermaid tackles this by not having it be applicable to all possible graphs (flow chart has a different syntax than gitGraph) allowing for a better domain specific language for each subset of functionality.
https://mermaid.js.org/syntax/packet.html is really neat... and it would be a pain to do that in GraphViz ... but I believe that's more of a demonstration that it might be better to do specific syntaxes for specific types of diagrams.
For graphviz, the "problem" with it is that it truly is general purpose and trying to make a general purpose syntax for diagrams is difficult. Mermaid tackles this by not having it be applicable to all possible graphs (flow chart has a different syntax than gitGraph) allowing for a better domain specific language for each subset of functionality.
https://mermaid.js.org/syntax/packet.html is really neat... and it would be a pain to do that in GraphViz ... but I believe that's more of a demonstration that it might be better to do specific syntaxes for specific types of diagrams.
The most general Mermaid form is https://mermaid.js.org/syntax/block.html - and that syntax doesn't really feel like it is an improvement over GraphViz.