This is probably a marketing bug on our part. While we did want to design for TypeScript from the outset, you can definitely happily write and publish plain JavaScript code on JSR. We probably need to do a better job explaining and featuring this.
If you're building something that is designed for TypeScript, made for TypeScript and publishes TypeScript sources, then I find it really hard to understand why you'd name it "The JavaScript Registry".
An npm compatibility layer doesn't sound like it has anything to do with TypeScript? I'm assuming that you can use JavaScript anywhere you can also use TypeScript.