What is the meaning of "ruined" in the poem?
Generally speaking, the word "ruined," when applied to young women in Hardy's time, means the women had been morally or sexually "spoiled" by becoming prostitutes or mistresses of wealthy men or by finding some other way of monetizing their sexual desirability. The usage suggests passivity; ruin is something that is done to women by men. It also objectifies women, in that their "ruin" is a fall from an earlier, better state that any (male) outside observer could see. Hardy calls attention to this usage by the form of the poem and the repetition of the word "ruin" at the end of each stanza.
The poem takes the form of a dialog between the "ruined" woman and a female friend who chances to meet her in London. There is a certain infantile quality to the structure of the poem -- the repetition resembles a nursery rhyme. The friend notices a different thing about the "ruined" girl's appearance in each stanza, about her dress, her speech, her hands, and so forth, and the ruined girl ironically attributes these refinements to being "ruined." The ordinary sense of this is that her "ruination" is a perversion of an earlier, purer state, except that it is pretty clear from the poem that she is, at least materially, much better off now. So, while the poem on the surface appears to condemn her "ruination," at the same time there is a recognition that the only path to prosperity for girls like these lies through "ruin." In this sense, the "ruin" in the final stanza is not something that has happened to the girl, but something she has chosen:
— "I wish I had feathers, a fine sweeping gown,
And a delicate face, and could strut about Town!"
— "My dear — a raw country girl, such as you be,
Cannot quite expect that. You ain't ruined," said she.
Her friend expresses envy at her coming up in the world, and the reply "You ain't ruined," could mean regret about her state, or it could be a suggestion, along the lines of, "If you like all this, you should become 'ruined' too!" I think it is entirely characteristic of Hardy to mean both things at once.
In Thomas Hardy's poem "The Ruined Maid" each stanza ends with a line that refers to the speaker as being "ruined." The word means that the woman has been morally spoiled and tainted, specifically, she has become a prostitute. The poem uses the word with great irony; although a woman who is "ruined" should feel regret and despair, the speaker seems satisfied and even smug with that appellation. Previously, she had worked at hard labor doing field work; her clothes were tattered and she had no shoes because of her extreme poverty. Her speech and hands were coarse. Now she returns dressed in finery, speaking in language that "fits 'ee for high compa-ny," and wearing lovely gloves. Even her complexion is "delicate" now. Her explanation for these great changes in her appearance and demeanor is that she has been "ruined." In other words, she has gone into prostitution as a way of escaping the extreme poverty and the lack of opportunity that faced women at the turn of the 20th century. The reader is left wondering what really "ruins" women.