I've never thought about the life cycle of mimics before, but it makes sense that a creature that imitates a treasure chest might start its life imitating single coins and other small bits of treasure.
Perhaps they birth thousands of young at a time, and what looks like a pile of gold is actually a brood.
DM: “How do you want to do this?”
Player: “ I slice the dragon’s cockles with my long and shiny blade”
DM: changes song, plays even more intense battle music
Player: “What the-“
DM: “Now it’s time to fight the horde! Roll initiative”
I love when in RPG video games after defeating the dragon you can see on your screen a mountain of gold, you click on it, and there's like 200gp and a sword +1.