Refusing to bathe is a way for Liesel to cling to and acknowledge the past, especially the people she has loved and lost. On the night she first arrives at the Hubermanns, for example, she refuses to take a bath:
There was no way she was getting into any bath, or into bed for that matter.
This refusal to bathe symbolizes Liesel's resistance to the idea of settling into a new home. She is frightened and wants to be with her mother. When she does finally take a bath after two weeks have passed, this represents her acceptance of life with the Hubermanns.
At the end of the novel, in the Epilogue, Liesel again does not bathe. This is, as demonstrated earlier, her way of honoring the dead and not letting go of them. She wants to preserve the ash from the Himmel Street bombing on her skin in solidarity with those she loved who died. Finally, when Liesel is ready to let go, she walks into the river where Rudy rescued the book. This mirrors her bath after two weeks at the Hubermanns.
The symbolism of bathing or not bathing shows us that Liesel is a resilient person. She will remember the past and mourn what she has lost, but she is also capable of moving forward after a period of grief.