Well, mermaid diagrams are "just" a list of nodes and their relations. You'd expect any llm capable of generating code to be able to generate them

Writing an SVG of a pelican riding a bicycle without being able to see the result and iterate based on that is incredibly difficult by comparison. I'm sure some humans could do it, but I sure can't. That's part of the beauty of it: it's very difficult to do but a toddler could judge the results

Writing an SVG of a diagram by hand would be somewhere on the middle ground. Or depending on the number of nodes might be even harder than the pelican. Layouting diagrams can get tricky very quickly. It's also one of Mermaid's biggest weaknesses