Margot is different from the other children because of her looks, her personality, and her experiences. Margot is "frail," and she is fair-haired and white-skinned, so much so that she looks colorless, like a washed-out photograph. Margot is quiet and withdrawn--she doesn't have the boisterous personality that many of the other children have. Her voice is soft, and often she doesn't speak at all. She keeps her distance from the other children rather than joining in their antics. In fact, she is a very sensitive girl who seems to have some deep-seated emotional issues. When she screamed when the water touched her in the showers, that confirmed to the others how odd she was. Because she can remember living on Earth where the sun shone often, she finds the constant rain on Venus oppressive, and she seems to be depressed. That's why her parents plan to send her back to Earth soon. She doesn't fit in on Venus.
Despite all those differences, the one thing that seems to set the children against Margot more than any other is that she has experiences they don't share. All the other children have a homogeneous background: They have been raised on Venus and know nothing of life outside the underground complex they live in. That Margot remembers seeing the Sun and that she knows about life on Earth first-hand makes the children jealous of her, even though Margot doesn't act like a know-it-all. Beyond that, the children know that she will have a chance to go back to Earth soon, a chance that evades the others. Her past experiences and her future plans set her apart from the others.
Why the other children dislike Margot is a strong theme in the story. Bradbury creates a scenario that allows modern Earth-bound readers to examine their prejudices. Margot represents the "other," and human beings instinctively despise those outside their own tribe. Perhaps her rich and varied experiences caused them to wish they could escape their underground home, so they became jealous. The fact that she wouldn't join their games might feel like an insult to them, so they lashed back to give her pain. But part of their dislike stems from a simple lust for power: Margot is weak and alone; they are strong and have numbers on their side. Such a condition spurs bullying, and that's what happens in the story.
Although the story is overtly about children on a different planet in the future, it makes all readers, children and adults, think about how they treat others and whether they allow prejudices to mar their behavior.