This isn’t so much a song about how the grass is always greener, but more of an observation about how much we deal in extremes as a society. Time and again you see a person built up in the media and social media, for example a musician or sportsperson, only to be torn down by the same outlets who built them up.
Simply being so entertaining/good as a sportsperson or musician, good enough to be in the top <1% of your field, doesn’t mean they automatically are also equally as “good” as a person. Sure, we’d all (most of us) like to be better people, but a lot of people are huge pricks and a number of them are probably musicians and sportspeople. Some of them very famous for being good at what they do. But still pricks.
Superstar is about how people are classified as an absolute when virtually nothing about humanity is absolute.