This is such a great question. While there are severe social and political strands present in the short story, there is another moral thread present about what constitutes happiness. The question that LeGuin seems to pose is whether or now an individual can be happy, knowing that there is suffering present amidst their sense of happiness? A larger implication of this is whether or not someone can be happy knowing that their happiness comes at the cost of another? When we think about how the role of the subjective has changed over time, the Romantics are some of the most resonant voices to enter our collective psyches. The Romantic thinkers argued that individuals must "seize the day, " embrace carpe diem in the idea that they are able to fully live life in accordance to their own subjectivity. They combined this with a belief that human happiness and subjectivity will merge with collective happiness. Yet, with the growth and proliferation of industrialization and commerce combined with its expansion both within and outside national borders, it seemed as if the espousing of human freedom was all that was heard about the subjective. The modernists might have grasped this all too late, and the result was a social order where others were happy, but at the cost of another's misery. The grandest of homes were erected and the most magnificent of marvels created, yet done so at the hands of others' suffering or existing along with the most dire of conditions adjacent.
This is LeGuin's rendering of Omelas. Le Guin does not offer any easy answers. Omelas only exists because of the suffering of the child. If the people of Omelas free the child, then their happiness is lost and the child, itself, would not know what to do as it has only known this condition abuse and neglect. Freeing the child, then, accomplishes nothing because the child won't be able to revel in its newly established state and coupled with the emotional destruction of Omelas, leaves the individual responsible for spreading more unhappiness. One cannot absorb nor comprehend the condition of the child and go back to Omelas because guilt is not present. This means, that if one wishes to acknowledge the voice and experience of the child, separation and alienation from Omelas is the only option. It is such a vision of human action that compels the reader to fully grasp that consciousness in the modern setting is one where individuals have to be fully ready to accept that the presence of unhappiness is something that wraps all of our visions of "happiness" together. It binds them, bifurcates them, to quote Borges. There is little way to escape or extricate one from this reality: An individual's happiness is only subjective and ends at the nose of another whose suffering can be both endless in scope and/ or even caused by the joy of another. There are no easy answers here, only the reality that unhappiness might be the child of happiness and that within this painful schizophrenic contexts is where humans reside.
LeGuin's disclaimer, "How is one to tell about joy?" leaves the defining of happiness to the reader who becomes involved in the creation of an alternative reality. With her suggestions of adding something or taking away--"it doesn't matter," and "for those who like it" draw the reader into the moral responsibility as a creator of the utopia.
So, what does the story say about happiness? It seems to place the responsibility of creating happiness into the realm of the reader. Thus, it becomes an existential question as the reader must decide if happiness is contingent upon a scapegoat or if one must create one's own happiness out of the ashes of one's own misery.