Junko would insist on purifying the metal first before letting anyone or anything touch it. Then she'll be disappointed because this "metal" isn't literal metal as in substances such as tin and iron, rather, it is a genre of music. So she lets Clownpiece go ahead and do whatever she pleases with it.
And the metal music Clownpiece makes ends up so (deliberately) terrible, it drives everybody nuts.
What if Gensokyo was really a realm dreamed up by Maribel and that every single thing and incident in it was just part of her imagination? ((That might explain why Yukari looks a lot like her, she (Maribel) invented it after all, so it makes sense that she would design the guardian of the universe after herself.))