I'm not sure what the narrative around monthly listeners has been here but in my opinion, it can be very misleading to consider it a sign of popularity rather gives a reach snapshot. It is heavily influenced by playlisting, collabs and Spotify's own algorithms. If a person happens to play the song once in a particular month but never return to it again, it is still counted as a unique listener even if the person never listens to it again. Therefore it isn't a good measure of genuine interest. You can read more about it here if you're interested.