Here's why she is called Ashputtle:
Cinderella's step-mother made her do all the unpleasant and difficult household chores. As a result, Cinderella had to spend most of her day cleaning the ashes and hearths around the kitchen fire which resulted in her clothes always covered in dust, dirt, and ashes—she looked 'cindered'. Cinderella received the harsh nickname "Aschenputtel" or "Cinder Maid or Ashey Paws" due to her perpetual dust and ash coat from cleaning fireplaces and soot. Eventually, people just began refer to her as Ashputtle only without her real name Cinderella attached. That term eventually became her alias during this period of mistreatment in servitude