If someone is sassier, they are more confident and a little disrespectful in a fun way.
Being sassier can be seen as funny or annoying, depending on the situation.
Examples:
She is sassier than her sister.
He became sassier after he won the game.
The cat looked sassier than the dog.
The little girl got sassier when she put on her sunglasses.
The monkey became sassier after it put on a hat.
The bird was sassier than all the other birds.