。
Her handwriting is amazing.
Literal
She [topic-は] writing [subject-が] amazingly skilled.
うまい is the more colloquial cousin of 上手 — both mean 'skilled, good at,' but うまい is plainer i-adjective register, while 上手 is a な-adjective with a slightly more formal feel. すごく ('amazingly, terribly, very') is the casual intensifier — also commonly heard as すごい (technically the i-adjective form, often used adverbially in casual speech). The combination signals informal, peer-level praise. うまい has a second meaning ('tasty, delicious') that's just as common in food contexts; here the 字 ('handwriting') subject locks in the 'skilled' reading.