What would make it different if Omelas was not a Utopian Society?
I can understand your confusion. Omelas does not seem like a utopia because they are sacrificing a child. However, it does qualify. It is utopian because the people are happy. The conditional description by the author which fails to develop the "perfect" parts of the society is necessary. She is focusing only on the central core of utopia, that the people who live in live happily.
This is necessary to the story. The people may have to sacrifice a child, but it is rationalized away because all their life is happy. They are never scared or depressed or in pain. The sacrifice seems reasonable.
If the setting wasn't utopian, then Le Guin could not make the title work. The important event of the story is that there are some citizens who do not feel that life long happiness is worth the sacrifice of a life. This is a powerful universal message, and gives hope to human nature. Faced with a choice, some would give up utopia for their moral beliefs and the sanctity of human life.