It is important to realise that the final paragaph of this chapter needs to be read in conjunction with the penultimate paragraph, where Mrs. Tulliver weakly complains to her husband about the way that he is treating Maggie and indulging her:
“I wonder at you, as you'll laugh at her, Mr. Tulliver,” said the mother, with feeble fretfulness in her tone. “You encourage her i' naughtiness. An' her aunts will have it as it's me spoils her.”
The final paragraph seeks to explain the "feeble fretfulness" with which she speaks by providing the reader with some sort of context about her character. The narrator tells the reader that although Mrs. Tulliver grew up as a "good tempered person," that good temper soured as she became more and more peevish. The narrator contrasts the character of Mrs. Tulliver to the pictures of Raphael, looking at Madonnas with their babies. Just as the Madonna in reality would have lost their complacent tranquility as their child grew up and became more challenging, so Mrs. Tulliver became less and less "good tempered" as her children grew up. This is reflected in her "feeble fretfulness" and the ineffectual way in which she tries to mother her children.