。
She's a second-rate singer at best.
Literal
She [topic-は] at-best second-rate [genitive-の] singer is.
せいぜい ('at best / at most') sets a ceiling on a judgment — the speaker is saying her peak is right here, not above. It often pairs with limiting predicates and is felt as cool or cutting. 二流 ('second-rate') uses 流 ('current / school / class') as a tier counter: 一流 ('first-class / top-tier'), 二流 ('second-rate'), 三流 ('third-rate'). The の links the rank attributively to 歌手 ('singer'). Together the sentence is dismissive — 二流 in Japanese, like 'second-rate' in English, is closer to 'mediocre' than to 'second-best'.