[Opinions] Re: Why did some names more/less "classic" than the other, even though the peaks were around the same time?
Some of the names had more use in earlier periods, even if they didn't peak as dramatically, which gives them a more "timeless" quality.
Using your examples, the names Amy and Christina have longer histories than Angela, with more namesakes from other eras. So as a result, Angela feels particularly tied to one era of popularity, while Amy and Christina are less dated and more "classic".
I think it can be hard to get a real sense of how familiar a name was in culture, just using the popularity charts. There can be historical examples of people who loom large in cultural consciousness that give the name a more timeless quality - like Amy from Little Women, or Christina, the 17th century queen who became better known after a movie about her came out in the 1930s. I can't think of a culturally impactful Angela from an earlier era, so it's very 1970s.