Some diagrams use the same or very similar syntax, but I think most vary at least a little. PlantUML also supports a number of more esoteric diagrams than Mermaid, though Mermaid supports enough for most people (flow, class, state, sequence, entity, etc).

All said, I saw a recent comparison between the two where they do overlap, and they have very very similar expressive capabilities. One rendered a little better than the other in some diagrams, then vice versa for others.

But I'd say Mermaid being built directly into so many markdown apps now--and not requiring a client/server architecture (or a .jar file)--probably nudges me in its direction.