M.C. definitely doesn't get the girl. But that's okay because he's building a wall that's going to save his home from a sliding spoil-heap.
And it's not just him—the ending is all about M.C. empowering the kids around him to do something truly purposeful and meaningful. His younger siblings get in on the action and so does Ben Killburn:
There began to take shape a long, firm kind of mound. The children fed it. M.C. shoveled and Ben packed it. In the immense quiet of Sarah's Mountain late in the day, they formed a wall. And it was rising. (14.237)
See? No adults. The whole ending only mentions the kids in the book. And it's kids, not kid—the ending is specifically not just about M.C., which, by the way, makes the ending the exact opposite of the title (See: "What's Up With the title?"). It's about a group of kids working together as a community for a meaningful cause that benefits said community, which in this case, is the Higgins' home.
Which is why the sentence right before the last paragraph is "Good-by, M.C., the Great" (14.237). That loner/individualist/egotist is gone. In his place, the hard, nose-to-the-ground worker who inspires by example has emerged. M.C. is actually pretty great at this point.