There are two main tacks Hardy uses to present Sophy's tragedy. The first is that he presents her as innocent of blame. The second is that he presents her as a victim of the Twycotts (name meaning: two + cold, hard man). The one shortcoming Hardy gives Sophy is that she is not a romantic with deeply passionate feelings. It is difficult to tell whether Hardy thinks this a flaw or not. When asked by Vicar Twycott if she wants to marry Sam, she says, "Not much," denoting a lack of intense emotion. Later, when Sam comes upon her notice again after she is widowed, Hardy presents Sophy with the same cool emotions.
She has thought of Sam over the years, not "passionately" but now with "tender interest." It seems likely that rather than this being a flaw, it is simply her personality and the well-established reason that she does not stand up to Randolph: she is not a passionate person who exerts greatly in any direction. This adds to the tragedy of her story: a peaceful, gentle natured woman is dominated in succession by two hard-hearted men.
In the early part of the story, Hardy establishes Sophy's blamelessness by showing her moral and upright inner character traits. She is respectful of Mr. and Mrs Twycott and of the day of Mrs. Twycott's death. She reprimands Sam for being frivolous when she and her employers are in mourning. She rescinds her acceptance of Sam's proposal when they quarrel (though we have no idea of what they quarrel about, Sophy is presented as having a rational reason, not an emotional one, for calling off the engagement). One important step Hardy takes toward establishing Sophy's blamelessness is in name symbolism. Sophy, wisdom, is above blame though Twycott was not, which leads to the second tact of presenting her as a victim.
Sophy: Greek; wisdom
Twycott: Old English, twy + cott: two + cold, hard man [Vicar Twycott and Randolph Twycott]
Randolph: Old English, wolf + shield; loyal + defender
The names put the blame for victimizing Sophy on the cold, hardness of the two Twycotts. Though the vicar loved Sophy, he was ashamed of her, as is proven by his move to South London, and he allowed Randolph to be disrespectful to her, as is proven by the introductory incident with the "confused ideas on the use of 'was' and 'were'."
Mr. Twycott knew perfectly well that he had committed social suicide by this step, despite Sophy's spotless character, and he had taken his measures accordingly.
Randolph, her wolf shield, was meant to be her loyal comfort and joy and her defender (surely a beloved son would adore her and not care about a beloved mother's abilities with 'was' and 'were'). Yet his cold hardness of heart overpowered the hope she had for him and he became her oppressor and she his victim. Since Sophy has consistently been presented as mild and even-tempered and not given to emotionalism (even in her unhappiness when she wondered "if life in a cottage with [Sam] would not have been a happier lot than the life she had accepted") that she would defer to her son and put his comfort above her own is perfectly in keeping with her inner traits and the reason she was so victimized.
she seemed to be pining her heart away. 'Why mayn't I say to Sam that I'll marry him? Why mayn't I?' she would murmur plaintively to herself when nobody was near.