What is a "lottery"? How does the title lead you to expect something very different from what the story presents? In what sense is the story's title ironic?
Nowadays a lottery usually involves NBA sports teams who have had losing seasons who get the first three picks of the draft. Or a lottery is a drawing for a prize or money such as the state lotteries in which people can win millions of dollars. At any rate, the word lottery has positive connotations. But in earlier times the idea of drawing lots (from which the word lottery derives) was usually a fearful experience.
For instance, the Jews of the Old Testament and the disciples of Christ prior to Pentecost cast lots in order to determine the will of God. Lots were sometimes marked sticks or stones with symbols put on them that were thrown into a small area and the result interpreted.
"The lot is cast into the lap, but its every decision is from the LORD" (Prov. 16:33).
There are many instances of casting lots in the Bible.
Also, in ancient times, lots were cast in other cases, including the selection of a scapegoat. (Lv 16:8-10)
With the modern definition, a reader is really surprised by the purpose of the lottery in Jackson's story; in such a case, the term is certainly ironic.
But with the ancient purpose of obtaining a scapegoat, the outcome of the lottery is in line with the stories of the Old Testament; nevertheless, it is still ironic that this denotation and purpose of the lottery would be used in a modern story and Mrs. Hutchinson be made such a scapegoat:
Bill Hutchinson...forced the slip of paper out of her hand. It had a black spot on it, the black spot Mr. Summers had made the night before with the heavy pencil...and there was a stir in the crowd.
"It ain't fair, it isn't right," Mrs. Hutchinson screamed, and then they were upon her.