The Heart of Darkness is held by some to be a racist story, but in my opinion it is not. There is a famous essay by Chinua Achebe, which holds that the story is racist, and I have provided a link to that essay for you. Whether or not the story is racist is a judgement you will want to make for yourself. Quite often, a literary text will contain evidence to support different points of view, and each of us brings a different perspective to a text, as well.
From my perspective, if you read the story carefully, you will see that it is the colonial British who are painted as incompetent, ineffective, and inhumane. Their machinery is rotting, they have no idea how to live in this land, and they manage the natives with casual cruelty. They are portrayed as being rigid and maladaptive. In short, what is shown is how foolish and uncivilized the white man is. Most of the negative portrayals of the natives show the white man as the cause of the problem.
If you are expected to write an essay or even a brief response to your question, you will want to go through the story and ask yourself whether or not the scenes are providing a negative portrait of the natives or a negative portrait of the British. Use the text to support your opinion.