There are several reasons that Sophy is characterized by Hardy as a victim in this social protest short story (Hardy was a great champion against what he saw as entrenched social injustice toward women and he took the brunt of significant social protest for his position). The principle reason is disclosed in the title: "The Son's Veto." This title shows the ironic power of a son over his mother and also shows the unquestioning power a male has as represented by the finality of the word "veto."
Randolph makes his mother into the ultimate victim--a process that we have seen in progress since the opening of the story when he chastises Sophy for her dialectal grammar (grammar native to her native dialect as opposed to grammar ordered by the Standard London dialect of English)--when he, her child and beginning when he was still a schoolboy, protests her intention of adding some happiness to her life by marrying Sam, the grocer, after she became a widow. We can only sigh with regret that Sophy's victimization had such a long history that she felt powerless to exert her own authority as mother and sole parent to make her own independent choice.
The ultimate demonstration of Randolph's victimization of Sophy comes when he forces her onto her knees to swear that she will never marry Sam. The final demonstration of his success in victimizing comes when he rides atop her funeral hearse as he accompanies her lifeless body out to her final resting place, passing Sam en-route (one always wonders how Sophy won that stroke of independence from her men, perhaps she specified her burial place in her will signifying that only in death could women in Hardy's society be free from male victimization).
Sophy's victimization began when the vicar moved her to the south of London in order to disguise her village roots and to find a society that they might be welcome in. This act, though it might seem to have been for Sophy's welfare, when woven into the disdain the vicar allowed to grow in his son toward his mother begins the etching of a subtle picture of continual victimization. So the direct answer to your question, "Why is Sophy a victim?" is that she was a victim because the men in her life, the vicar and their son Randolph, disdained her origins and her qualities and repressed her where possible, hid her otherwise and ultimately forced her into allegiance to them and them alone when by rights, she might have made her own choices had she not been bound legally (the Vicar's will) and spiritually (Randoplh's insistence upon swearing) from doing so.
[Vicar] Twycott knew perfectly well that he had committed social suicide by this step [of marrying Sophy], despite Sophy's spotless character, and he had taken his measures accordingly. An exchange of livings had been arranged with an acquaintance who was incumbent of a church in the south of London, and as soon as possible the couple removed thither, abandoning their pretty country home, with trees and shrubs and glebe, for a narrow, dusty house in a long, straight street, and their fine peal of bells for the wretchedest one-tongued clangour that ever tortured mortal ears. It was all on her account. They were, however, away from every one who had known her former position; and also under less observation from without than they would have had to put up with in any country parish.