The quote in question can be found in chapter 2 of Mockingjay. The full quote reads,
So I guess the trip to 12 wasn't a complete waste.
Katniss is referring to her trip back to District 12, which is narrated to readers in chapter 1 of the book. Katniss has returned to her district to see the remains of her hometown and the people who still live there. The entire area was in bad shape before Katniss left for the original Hunger Games, but it is nearly a complete wasteland after the Capitol's firebombing of the District.
Katniss is able to visit her home, and she is able to gather a few things of sentimental importance to her and her family. Near the beginning of chapter 2, Katniss returns to District 13 and gives Prim her cat. Katniss gives her mother a book of plants and a wedding picture. Prim and her mother are consoled by the return of their items, and seeing them this way gives Katniss a moment of comfort. Her trip to District 12 was not a fun trip, but it did give her the chance to bring those items back to her family. Therefore, the trip wasn't entirely bad or a complete waste of time.