。
She's very smart, and on top of that, she studies hard.
Literal
She [topic-は] very head [subject-が] good [on top of-上に] all-out studies.
~上に attaches to a complete clause and stacks an additional, reinforcing fact on top — like 'on top of that, what's more.' 一生懸命 ('with all one's might') is set in the four-character compound mold and pairs naturally with verbs of effort (働く, 勉強する, 頑張る). The conjunction here pushes both qualities in the same direction: smart AND hardworking, doubling down on the praise.