。
My mother is a good cook.
Literal
I [of-の] mother [topic-は] cooking [subject-が] skilled is.
Classic 上手 pattern — the skill (料理) is the grammatical subject, the person (母) is the topic. 母 is the humble form for one's own mother; お母さん would be used for talking about someone else's mother or when directly addressing. Using 母 with です retains politeness without elevating one's own family.