If the mother has mild mental retardation, will her child be retarted also?

Depends. If there is a known genetically determined cause, it is possible.
Not necessary. It depends on the cause of the mother's retardation.
Possibly. There is such a thing as familial intellectual disability, often associated with small head size. There are many genetic syndromes that cause intellectual disability and are inheritable, so mom could see a geneticist to try to determine if she has one of these. Mom also can receive in-home help with child-rearing and activities of daily living through state agencies that receive federal funding.