Jane Austen's Emma is, like most of her novels, a strongly-stated commentary on the social mores of the time. One of the strongest themes is that of integrity. Emma conducts herself with integrity most of the time, but when, in a moment of impatience, she blurts out a cruel remark to a friend, the people present are shocked and the target of the remark is humiliated.
An additional theme illustrated by this occurrence is that of the unspoken rule of polite society to behave kindly towards those in less-privileged social classes. The woman insulted by Emma is of a lower social class, and so it is seen as doubly cruel, since she cannot easily rise above her station, and Emma, as a well-bred woman, is expected to behave in a more genteel way.