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The Marshmallow rim was obtainable in Season 19: "Freezing Cold" by reaching level 3.
Supply is widely distributed — no single holder owns enough to register as a hoarder.
Marshmallow has an mu of ~68, which is actually VERY low for an item with just the 'rare' rarity. For example, the Snowflake rim with the 'very rare' rarity has mu of ~108. Another example is the Planetary rim with mu ~139. Suggestion made by: @abedaape