。
She gave me an awkward smile.
Literal
She [topic-は] me [to-に] unnatural smiled.
不自然に ('unnaturally') is the adverbial form of 不自然 ('unnatural'), the negation of 自然 ('natural') with the negative prefix 不~. Applied to 笑う ('smile, laugh'), the adverb suggests forced, awkward, or strained — not a genuine smile. The に here pairs with 笑った to give the recipient (the speaker) without a benefactive ~てくれる, leaving the action neutral or even slightly negative — she smiled at me, but in a way that didn't ring true. 笑う covers both 'laugh' and 'smile' in Japanese; the difference comes from context and other markers (微笑む is specifically 'smile').