。
She's by no means a beauty.
Literal
She [topic-は] never beautiful-person is-not.
美人 specifically means 'a beautiful woman' — the kanji are 美 ('beauty') + 人 ('person'), but the term is gendered. The masculine equivalent is 美男 (びだん) or イケメン (slang). The sentence flatly denies that she falls into that category, with 決して supplying emphasis. In Japanese, calling a non-美人 ふつう ('ordinary') or even 個性的 ('distinctive, individual') often reads as a polite hedge — direct denial like this lands more bluntly.