Great Expectations is a story of the triumph of good over evil, one in which the fantastic dominates over realism. Justify your agreement or disagreement with this observation.

I would agree both that good triumphs over evil in this novel and that the fantastic—I might call it the unlikely—dominates over the real.
Two fantastic events from Pip's childhood come to dominate Pip's adult life. The first fantastic occurrence is Pip helping the escaped convict Magwitch, who is hiding in the fens. This is an unlikely and melodramatic event. Even more fantastic is that a convict would remember Pip and spend his life working and living rough to set Pip up as a gentlemen living the high life. This is one very good-hearted man!
Miss Havisham at Satis Hall is another fantastic character, a fairytale-like figure of a witchlike woman isolated in her "castle" and trapped in the past. The other significant event in Pip's childhood is Miss Havisham inviting him to her home. If Magwitch is good and remembers the past to do good, Miss Havisham is badly damaged by having been been left at the altar and remembers the past in order to do harm.
Both these fantastic characters have a significant impact on Pip's adult life, but the mutual impact is to soften his heart and turn him from a person who bases his life on snobbery to one who can begin to discern true goodness in people.