She was never a diplomat. She was the Rebellion's spy in the Imperial Senate. She traveled on a consular ship because she thought it would keep her immune from search and seizure. Her "mercy missions" were intelligence operations. After her cover was blown, the Senate was dissolved, and her planet was blown to smithereens, she became kind of useless. The military elements of the Rebellion--all humans, probably Alderaanians--kept her around, but they didn't send her out on diplomatic missions, a task for which she seems temperamentally ill-suited.