Question: What do you expect when a book totally comes from inside the head of a thirteen-year-old boy who isn't exactly getting things to go his way?
Answer: Things get heavy pretty quickly.
Just check out this conversation between M.C. and his father:
It wasn't often that he and Jones could sit down together without Jones having to test him or think up a game to see if he could win it. He knew Jones only wanted to have him strong and to have him win. But he wished his father wouldn't always have to teach him.
Just have him listen to me, M.C. thought. Have him hear. (4.27-28)
M.C.'s always thinking, and the stuff he thinks about is deep and difficult, whether it's Lurhetta's safety or the future of his family. And since our narrator stays right there with M.C. throughout the story, a super-pensive feel permeates the whole book.
You know how M.C.'s only thirteen and life as he knows it is threatened? Sophisticated though he might be in plenty of ways, this means he's also a bit on the melancholic side from time to time. In his defense, though, when you don't get the girl that you like and a spoil-heap threatens your home, it's hard to feel chipper. Check this out:
The hurt of [Lurhetta] going pressed in on him, like the thinning fog. High up in the air, he swung his pole in its sweeping arc. He thrust the knife at forming clouds. The fog was lifting far off on the Ohio. So M.C. stabbed the river and cut it in two. He sliced off chimneys of the steel mill, barely visible. And he cried out once as his pole swayed and swooped, chopping up the mist-shrouded town of Harenton. (14.110)
See what we mean? M.C.'s trying to cut fog up. It's futile and sad, and it's all about loss. So yeah, the tone is melancholic.