That is a good thesis, but it would be best to explain how the perspective allows the tone and atmosphere of the story to appear one way and then to, suddenly, change into something more Gothic and horrific at the end. It is the combination of perspective and atmosphere what provides that traditional idyllic setting. That same combination will, as you say, render the horror that is found later.
The third person omniscient is a point of view that can be used broadly enough to effectively convey the actions and emotions of every important character in the story. Notice how in "The Lottery" the narrator does not directly tell the story of Tessie from the beginning until the end of her last day of life.
Rather than emphasizing on one storyline and one person, the narrator flashes from one character to another pointing out how each of them prepares for the lottery. The omniscient perspective also helps to define the role of each of the townsfolk in the process of the lottery, giving equal importance to the parts that each of them play in this dreadful practice.
The point of view also allows for the narrative to go back and forth in time. Notice how the narrator tells about the history of the lottery, how it has changed over the years, and its prospects for the future.
All this being said, it is a good thesis to add that it additionally helps set a tone of neutrality and unsuspecting normalcy to what will latter on turn out to be a tale about murder, injustice, and barbaric practices.
Here is what I came up with: The dramatic third person point of view is effective in rendering horror in the midst of the traditional idyllic setting of the small and common village in Shirley Jackson's "The Lottery."