Depends what you call idiomatic. I dont think JSX is really a JavaScript idiom, more like a HTML idiom applied to JavaScript which happen to be often used together.*

At the end of the day, I think that calling something idiomatic is subjective and so, varies from people to people.

*: to be clear, my point is many people use JSX with React because it feels idiomatic (I manipulate HTML/DOM, so why not just write with HTML-like syntax?) but it is not idiomatic to JavaScript and I don't think the other React APIs are that nice to use. If I didn't use JSX with React, I'd tend to use other framework because, in my opinion, they make more sense.