Is it possible to use JavaScript instead of Dart on Flutter to make cross platform mobile apps?

Yes you can: https://github.com/openkraken/kraken I think this project is from Alibaba, and with it you can use js instead of dart, and then it will be rendered by flutter.

No, you can't. Flutter won't "compile" JS code, it is possible to use platform native code however (Swift, Obj-C, Java, Kotlin) to specific tasks.

Even if you want to use Flutter to create a web application, you need to write it in Dart.