The speaker of "Lady of winter" seems to be describing a woman, perhaps herself, whose beauty is tied to danger and illness. The first lines pile up unflattering comparisons: pretty as sin, pretty as sickness. Beauty here isn't sweet or innocent. It feels a little guilty, a little unhealthy. Then comes a shrug: "I do not care." The speaker refuses to be bothered by what that kind of prettiness might say about her.
The turn comes in the second stanza. She is "glad" to be pretty, but only "as the ice." That image is cold, hard, and bright. Ice is lovely to look at, yet it doesn't need anyone's approval, and it isn't soft or warm. The title fits this, since winter is the season where ice belongs.
The mood is cool, defiant, and quietly proud. The short lines and plain words add to that feeling, like a calm statement made without apology.