。
She kept smiling the whole time.
Literal
She [topic-は] the-whole-time grinningly was-smiling.
にこにこ is a 擬態語 (state mimetic) for soft, sustained, friendly smiling — the kind of warm, low-key beam someone wears when they're quietly happy or pleased to see you. It pairs naturally with 笑う ('smile / laugh') even though the verb itself already implies smiling — the mimetic specifies the kind. Compare ニヤニヤ (smirking, often with mischief or bad intent), にっこり (a single gentle smile, not sustained), and 大笑い (full-on laughter). The polite past progressive ~ていました narrates the sustained behavior over a stretch of time.