Sansa's actions (and the overall character actually) are more clear when you contrast it with Arya's.
Arya is absolutely brutal, her literal words are that she is going to smile on Walder Frey as he dies and she makes him eat his own sons. Her characterization is far more problematic too, she is all good and caring with Lady Crane but goes absolute sociopath with Trant and Walder. It's almost unrealistic and cheap.
Sansa's words are only that anything about Ramsay will be forgotten, when she smiles she smiles to herself, not to Ramsay or anyone, it's very obviously a Littlefinger smile, she for the first time ever in her life has power over something. She even hesitates about watching Ramsay being devoured.
But Arya doesn't get half the shit Sansa gets as a character, I guess because she has a sword and is "badass".
edit:
This is the other thing, people act like if it was Sansa the one who physically tied up Ramsay and released the hounds, she actually asks Jon where is him, for all we know it was someone's else idea to put him there, even Jon himself.