。
She's gradually growing more beautiful.
Literal
She [topic-は] gradually beautiful [become-になって] [come-toward-てきている].
Three layers of aspect mark a slow visible transformation. きれい is a な-adjective ('beautiful, pretty, clean'), so the change-of-state form is になる ('become beautiful'), not くなる. ~てくる ('come to be / develop into') anchors the change as moving toward the present from the past, and the final ~ている marks that gradual transformation as still in progress. だんだん reinforces the pace as steady and incremental rather than abrupt. The whole stack is a reminder that Japanese has fine-grained tools for talking about how a state has been unfolding up to now.