Most readers find Dinah's saccharine piety distasteful; it seems like much too much of a good thing. To the extent that this reaction is based on an unhistorical view of the character, it is not valid. Dinah's style of speech strikes the modern ear as unbearably self-righteous and prudish, but a good part of this impression is based on the fact that her type of religious rhetoric has become cliched since 1800; we tend to consider it insincere and to parody it. But Eliot was creating a historically accurate picture, and this should be taken into account in any judgment of the character.
On the other hand, to the extent that the negative reaction is based on a perception of Dinah's personality, it is quite valid. Most critics would agree that Dinah is not a realistic character. She is an ideal, a perfect woman who has no faults, the personification of the abstract goal of Eliot's ethical system. There are no perfect women in the world, nor are there likely to be; even Dinah is rather blatantly non-human in her piety.
Dinah is a categorically good woman. She is completely dedicated to doing the Lord's work. Wherever she encounters suffering, she alleviates it, and she has spent her whole life in deeds of charity. Dinah's is not an abstract piety; she preaches what she believes and she practices what she preaches. Through long experience she has discovered how to comfort people and bring them to God, and in the course of the novel she has a good effect on everyone she comes in contact with. Dinah is also entirely humble; she sees herself as the servant of God's people. She is a working woman, and no task is too mean for her. She visits afflicted people in prisons and in their simple cottages, and she shares their hard life with them. She is absolutely unselfish. She never allows any thought of her own comfort to interfere with the performance of her duty, and she looks upon her own desires as temptations which must be resisted. She always places God before man, and other people before herself. And finally, she is totally benevolent. Dinah never does anything which is not likely to have good effects. She harbors no dislikes and treats everyone with respect and sympathy. No one in the novel, not even Bartle Massey, disapproves of her; she is universally praised, even reverenced, by all who know her.
Dinah gives up her life of exclusive ministering and preaching when she marries Adam. But she takes this step only after assuring herself that it is God's will, and her basic personality does not change at all. Dinah remains at the end what she was at the beginning: a morally perfect woman with the face and bearing of an angel and the heart of a saint.