It basically is — the same format is used for both defaults (app-defaults files in traditional Xaw programs) and customizations (.Xdefaults and the "resource database"). As far as I can tell, Tk picked it up from Xrm, and there actually is an option to have Tk use Xrm but I have not looked into that yet.

There is a certain inflationary use of the term DSL but maybe we should stick to stuff which at least attempts to be turing complete, like SQL or regex?

JSON and YAML aren't.

update

Well I stand corrected:

DSLs can be further subdivided by the kind of language, and include domain-specific markup languages, domain-specific modeling languages (more generally, specification languages), and domain-specific programming languages