まがい者 describes something or someone that looks genuine but is actually an imitation, making it a vivid synonym for “fake” or “impostor.” In the song, the phrase まがい者の天使様 means “you counterfeit angel,” accusing someone who appears angelic but is hiding their true nature.
FAKE is a song about losing at gambling and knowing full well you'll be back tomorrow. Janne Da Arc set it inside the narrator's head, where an angel and a devil are fighting, and the angel is hopeless at it: "あいかわらず天使は弱くて" (as always, the angel is weak). The devil holds out a hand, he takes it, and he admits he can't even count how many times he's been made a fool of: "バカを見る" is an idiom that means getting burned or ending up the fool, not literally "seeing an idiot".
The gambling details are specific and pretty funny. He bets on the dice: "ダイスが3の目なら" (if the dice come up three), then he'll call you an angel. The "銀の戦士達" (silver warriors) disappearing one by one are almost certainly his pachinko balls or coins running out, and under the flashy neon he mutters "「時間よ戻れ」" (time, go back). Luck itself becomes a woman he can't quit, "極上の麻薬みたい" (like the finest drug). My favourite line is the punchline aimed at her: "しっぽが隠れてないよ" (your tail is showing). Her devil's tail is sticking out, so she's "まがい者の天使様" (a fake angel), which is where the title comes from. He sees right through her, and he still can't stop: "やめられない" (I can't quit).