In all honesty I feel like both of them aren’t really used right. Like, people often use overrated to say that they don’t like a song/artist and nobody else should like it and people use underrated to say that they do like the song/artist and it should get more attention.
So it boils down to “like” and “dislike” but takes it a step further as if to say that nobody or everybody should like/dislike the song/artist.
In reality, calling a song underrated should mean it’s slipped through the net completely and barely anybody knows it at all (alot of songs/artists called underrated these days are popular songs/artists in their own right).
Calling a song/artist overrated should mean you are hearing about it too much and being underwhelmed by what it actually is, which doesn’t necessarily mean it’s bad, it just means it may be played on the radio so much and talked about so much that you just don’t care about it.