Yvette's red high-heeled boots are pretty flashy, especially for Central Europe in the seventeenth century. The boots are part of what Yvette wears as a prostitute. They are also of immense interest for Kattrin.
Based on what Mother Courage says about her daughter (VI, 124-126; V, 52-54), we figure out that Kattrin can't wait for peacetime, when she imagines she will get married and maybe have a family of her own. But when she plays with Yvette's boots and tries to wear them, later receiving them as a gift from her mother, this suggests that her dream of love is the fantasy of a child who's only ever known war.
Mother Courage gives Kattrin the red boots after Kattrin is attacked by soldiers and given a gash above her eye. The boots, which Kattrin used to covet, are now of no interest to her:
MOTHER COURAGE: […] There you are, all bandaged up. You'll get something, love, keep calm. Something I put aside for you, wait till you see. She delves into a sack and brings out Yvette's red high-heeled boots. Made you open your eyes, eh? Something you always wanted. Put 'em on quick, before I change me mind. Won't leave no mark, and what if it does? Ones I'm really sorry for's the ones they fancy. Drag them around till they're worn out, they do. Those they don't care for they leaves alive […] So it's a stroke of luck for you really. […]
Kattrin leaves the boots where they are and crawls into the cart. (VI, 246-263)
Courage tries to comfort Kattrin, but we can imagine that this only makes things worse for her. In effect, her mother tells her she's lucky the soldiers didn't kill her, in addition to attacking and (possibly) raping her. Kattrin should be lucky she won't be one of the pretty ones now, because they fare a lot worse.
So, are we surprised when Kattrin turns down the boots? What else could they symbolize to her now, other than a life of being abused, unwanted, and unloved by men? A life in which getting attention from men means violence or prostitution? Here, the red boots become a powerful symbol for the incompatibility of love and war.