Спокойной actually means "calm" rather than "good". But in English, we do not routinely wish one another a "calm night" - we would only say such a thing if we anticipate a serious risk of the night NOT being calm e.g. during a war. So "Calm night" would not be a suitable translation for what is simply a routine phrase on parting. It is used in the same way that we say "Good night" in English. So that is the appropriate translation.