No one knows for sure actually but many people believe it's based on controller input and what difficulty you play on. The more you move the more you earn. You also earn more coins by playing different modes.
One way of increasing your gold earning rate without cheesing the system is to up the difficulty rating. By taking on challenges which are above your Power Level, you’ll not only earn more EXP, but also gold coins too.
Super Smash Bros Ultimate awards gold coins based on controller inputs, rather than time played. Running, jumping, attacking and even ducking will incrementally increase the amount of gold you’ll earn at the end of the match.
The good news is that you can earn gold by playing just about every mode in Smash Bros. Ultimate. Classic Mode, World of Light, Smash Mode, Online, or Spirit Board will all earn you gold when you complete matches.
In Super Smash Bros. Ultimate, coins are earned at the end of each match based on the number of controller inputs registered by human players.
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