I agree that it's not a happy ending. It's bittersweet. 
Losing her sister was obviously tragic. As for killing Snow, her failure to kill him isn't sad. She knew the guy was going to die soon anyway, so there was no need for her to take his life. His life was already ending. Since there was no need for her to kill him, she chose to kill the next would-be dictator. 
The capital falling was "happy" in the sense that it was a positive thing, but revolutions are horrible events to live through even when they're righteous and successful. The happiness caused by (some) revolutions is rarely experienced by the revolutionaries themselves, who will always have any possible happiness tempered by their trauma. It's a gift revolutionaries give to future generations.
Katniss's children are the fortunate recipients of that gift; in them, she's able to see the joyful results of Panem's fall.