How do you describe an unfit mother?

Someone who cannot or will not provide proper care for a child.

Here are some links that can give you examples of an "unfit" parent.

http://courts.state.ar.us/opinions/2...25/01-380.html
http://courts.state.ar.us/opinions/2...5/04-1235.html
http://www.courts.state.md.us/opinio...2003/24a02.pdf
http://www.state.il.us/court/Opinion...ml/1012410.htm

Also See:
Forms of Child Mistreatment:
Forms of Child Maltreatment

From Barron's Law Dictionary - Unfit -
Unsuitable, incompetent, or not adapted for a particular use or service, 522 P. 2d 330, 334. Compare warranty [warranty of fitness]. In the context of a parent child relationship the term "unfit" usually, although not necessarily, imports something of moral delinquency. 321 P. 2d 190, 196.

It is best to contact your state's Attorney General's office - they can give you how your state defines "unfit mother".