For whatever reason, there's no way to see a song's difficulty in the item shop or your locker--the only place in the game you can see song difficulty at all is in the song picker on the Festival stage, and between songs during a session.
This annoyed me, and so I made a big spreadsheet organizing every Festival song by difficulty and instrument--this will help you shop for songs that are within your comfort level without having to keep googling stuff individually.
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