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.