No, you wouldn't use 'ça y est' in a restaurant to say you have finished your meal , I would use 'c'est bon, merci'.
Hope this helps!
I don't think that would be an appropriate response, particularly since it is a kind of expression used when something you've been waiting for finally arrives. You could, maybe, use it in an impolite way when the waiter brings you the dish you have been long waiting for.
-- Chris (not a native speaker).
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