。
Is dinner ready?
Literal
Dinner-of preparation [topic-は] become-done-[resulting-state-ています]-[question-か]?
出来ています is the state form of 出来る ('to be done, come into being') — 'is in the state of being done.' This stacks nicely on 準備 or 用意: 用意が出来ている ('preparation is done, ready'). The ~ています here is a resulting-state marker, not an ongoing action.