In Kotlin, you can use any Java library without a wrapper. That includes JavaFX of course.

TornadoFX is only a convenience skin on top of JavaFX to make Kotlin code more concise (but it's heavier also as it adds a lot of code to your application).

I wouldn't use Tornado - it's effectively dead. Hasn't been a commit merged in for 5 months, and doesn't seem any forks have really picked up the mantle.