。
She's a terrible cook.
Literal
She [topic-は] cooking [subject-が] frighteningly bad.
恐ろしく (おそろしく, 'frighteningly, terribly') is used as a hyperbolic intensifier — literally 'frighteningly bad at cooking.' This colloquial use of 恐ろしい as an intensifier is common in Japanese, similar to English 'terribly' or 'awfully.'