As David Gerard wrote in his 2017 book Attack of the 50 Foot Blockchain, "Cryptocurrency advocates and lazy journalists like to talk about the 'market cap' of crypto, which is the total number of coins or tokens in existence multiplied by today's price. This is a bogus number that's not actually applicable to anything — it's not money that was put into the crypto, it's not a realisable value like a company market cap, it doesn't affect prices — it's just an easily-calculated number that sounds good in a headline. Trading is so thin in any crypto, even Bitcoin, that you could never realise a fraction of the number."
That said, the public has been tracking market cap since the Dutch East India Company so hanging it on crypto is akin to ragging on Tesla because they publish horsepower ratings for their motors.
There is an argument that could be made about how crypto enthusiasts care about market cap rather than total money supply. "If anyone truly believed that cryptocurrency would supplant government-backed currency, they wouldn't use the metrics of speculation - market cap - instead of the metrics of geopolitics - total money supply" or something like that. But five comments in they're already whatabouting finance terms at which point you can solve the rest of the exercise as a facile circle-jerk.