Jugend Top can be an adjective if you're describing someone in a homosexual relationship. I doubt that was his intent, but it is an adjective in that instance.
No, in the context you are referring to, the adjective form would be the multi-word term "on top". "Top" itself can't be a proper adjective. "That person is very top" makes no sense, where if you were to use the multi-word term "That person is very on top" or another real adjective "That person is very purple", the statement makes grammatical sense.