Mermaid has explicit support for diagrams that look UML-ish (never mind the filled triangle arrowheads for inheritance): class, state and sequence diagrams. Graphviz doesn't do that easily.
PlantUML[0], which is built on top of Graphviz, does have support for UML and other services such as Gitlab do support direct embedding of PlantUML (which also means you can embed Graphviz, since PlantUML is an extension of Graphviz).
