The market cap of a stock (shares outstanding x share price) makes sense because each share represents a piece of the profits of the company and an ownership stake. If you took away half of the shares, the price per share would ~double and the market cap would remain the same since it's based on current cash + projections about the future profitability of the company, and the share price reflects those expectations. None of that is true for crypto.
Financially illiterate crypto people started multiplying the coin price x number of coins outstanding and calling that a "market cap" but there's nothing underlying the crypto world. Owning a bitcoin doesn't represent a percentage profit of some underlying economic activity that's being independently valued, coins are zero coupon bearer instruments, the value is in the coin itself. It's like talking about the "market cap" of pesos, just incoherent and easily gameable.