Sound is the primary sense associated with Kurtz. Why is his voice so powerful and important while his appearance less so?please, if you know the page or where can be the answer tell me it! thank you!
To find an explanation (I'm not saying there is an overt reference) my suggestion would be to go to that part of the story where Marlow first hears or comes into the same vicinity as Kurtz. That is often where characteristics are explained. So, to answer your main question, the fact that Kurtz' appearance is obscured speaks to the overall shadowy, unclear theme of the book. Darkness and obscurity pervades the novella, from the very beginning when the original narrator is sitting on the boat at the mouth of the Thames. Continuing on the journey, the view from the steamer is often practically non-existent, whether it is due to actual darkness or fog. This ties into the novel's overall message that things are not what they seem, that people are misrepresenting the actual impact colonialism is having on Africa and its people.
Hope this helps.