Market cap is a meaningful concept for items whose market liquidity is close to the total issuance. If all owners of Apple stock (or even gold) were somehow forced to sell all their holdings on market within one year, their total revenue would be fairly close to the market cap. That is not true for cryptocurrencies, not even for bitcoin.
I don't think that's right. Market liquidity doesn't need to be close to issuance, if people are choosing to hold for reasons other than lack of liquidity. Liquidity really only needs to be adequate for each holder or any group of holders operating as a group.
But for those scenarios the relevant quantity is market price, not market cap. Or, rather, (size of largest lot considered)*(mean price for sale or purchase of that lot).
The market cap is (market price)*(total issuance); why would total issuance matter, if only a small fraction of it can be traded at that price?