In the last few pages of the book, we see everyone who was shrunk restored to normal size, Marjorie's resizing machine smashed, Marjorie recruiting Arthur to be her new scientific assistant, the Ratbridge Nautical Laundry hosting a party to celebrate, and Grandfather and Willbury feeding weeds to the freshwater sea-cow mother who's been reunited with her now-resized calves. Arthur and Grandfather are going to live aboveground now, and Herbert, finally free, will live with them until he lands on his feet.
Willbury and Grandfather say that they rather like Ratbridge, even though it's not all perfect. Arthur agrees, thinking, "It's not bad at all" (55.102), and then he smiles. The end.
So we get a sense that the main plot threads of the book have all wrapped up: Snatcher's evil plans are foiled, Arthur and Grandfather are no longer exiled to the underground, Herbert is no longer a prisoner, Marjorie recovers her invention and stops it from being used to do more damage, and Willbury helps get his underling friends out of danger. Perhaps more importantly on a character-development level, we see Arthur go from a kid with an isolated underground life to someone who has a bunch of friends. He's even helped save the whole town.
It is, in other words, a happy ending.