I'm the Java Languages Owner at Heroku. While I can't reveal in full detail why we do (or do not) officially support a particular language, I can say that it has nothing to do with how resource-hungry the runtime is :)
Our determination to officially support a language is based on numerous factors including community demand, relationship with other technologies, suitability to the platform (C++ is really popular but not for web apps), and more. In any case, I wouldn't read too much into our support--it's not a determination on the language itself.
Fun fact: Clojure was the first JVM language supported on Heroku back in 2011 (even before Java itself)!
https://blog.heroku.com/clojure_on_heroku
Today, Heroku support Java (with Maven or Gradle), Scala, and Clojure buildpacks. JRuby is supported via the Ruby buildpack. Buildpacks are the mechanism that prepares each app for production, so in some ways our support is more tightly coupled to the build tools than the language. That's why we don't have a Kotlin buildpack (it can use the Maven or Gradle buildpack accordingly). In fact, there's nothing specially you need to do to run a Kotlin app on Heroku, so we have some docs and there isn't much more to say about it. We've also worked closely with the Kotlin folks to make sure the ecosystem plays well with Heroku.
Let us know if you have any trouble with Clojure on Heroku. Stackoverflow.com is a great place to start (and I personal try to watch for questions tagged with "Heroku" and "Clojure").