A traditional hero's journey often involves a witch-like character, and Milkman's has Circe. However, several things about his meeting with Circe are distinctly un-heroic. First there is the fact that, upon approaching the house, Milkman compares his experience to Hansel and Gretel approaching the witch's house. He portrays himself as a child, which isn't very heroic. On top of that, Milkman's encounter with Circe almost doesn't happen because he is denied by a door which doesn't have a doorknob that turns. Then, almost by accident (after he initially gives up), he is granted access to the house by his impressive foe and gets to meet Circe.
After that, on his way to the cave to find the treasure, he repeatedly takes the much more complicated route and ends up destroying his clothes. While his struggles on the way to find the buried treasure in a cave that was guarded by his father's skeleton might seem heroic, he discovered that he really didn't have to go to so much trouble to reach the cave. After he climbs the rock wall to get to the cave he notices that there was a much easier path up the hillside to reach the cave. Then on his way back from the cave (after discovering that neither the gold nor his father's skeleton were in the cave) he discovers a bridge across the river that he had tried (and failed) to walk across. The bridge leads to a path through the woods that he had struggled through on his way to the cave. Milkman is taunted by the paths that he should have taken, not a situation a traditional hero would find themselves in.
Milkman's story is framed like a hero's journey, but Milkman himself makes it impossible for anyone to take him seriously in that role.