Questions about diagrams IMHO can be on-topic if the specific focus of the question is how to use diagrams for things like
For example, a question like how can I draw round corners is not specific for software development, a question like is there a consensus to use round corners for the "Foo" elements in the UML diagram type "Bar" to express semantics XYZ could be on-topic.
In my point of view, the question in stake is more of the former type than of the latter (but YMMV), so I think it should stay closed.
Since the OP mentioned it in his "answer" here: a question like "I try to design a domain specific language for unit testing using ASCII art, but I have trouble with the semantics (or unambiguity) for this and that reason" could have been on-topic. Just asking how to improve the look for ASCII art, however, not.