Since it's called "The Ruined Maid," this poem must be about… a ruined maid, right? Yep, it sure is about a ruined maid. But what is a ruined maid, and what should a title with "ruined" in tell us? Hardy was a late Victorian writer, meaning he wrote his novels and poems towards the latter part of the Victorian period (which you can read more about here).
During this era, if a woman was "ruined" it meant that she had had sex before marriage—either she had become somebody's mistress, or a prostitute, or had just decided that she couldn't wait any longer. In those days (about the last 70 years of the nineteenth century), these women were what we would call "damaged goods," and Hardy loved to talk about them.
When we see a title like "The Ruined Maid" then, we know immediately that we're dealing with a poem about somebody who is morally questionable (at least according to the overly strict standards of Victorian conduct), who has compromised her virtue. A Victorian reader would have responded to a title like this the same way we would respond to a title like "The Prostitute."
While at first we assume that 'Melia is the ruined maid (she keeps calling herself that, after all), it becomes clear as we read that Hardy is playing with the definition of "ruined." 'Melia looks classy, clean, and well-dressed, while the other woman (we gather by implication) is dirty, tired, and worn-out from working all day on the farm (she never says that, but it is everywhere implied). In some ways, then, the other woman is ruined as well—physically that is.
The title of the poem sets up expectations that are mostly met, except for the fact that by the end of the poem we are left wondering just what exactly makes a maid "ruined," and which woman in the poem is the ruined one. (Hint: we think it's both.)