Any currency or "thing", be it fiat, crypto, or commodity, is given value by people who trust and believe in it. If you get more people to adopt and have faith in it, its value goes up. Once people stop believing in it and seek to escape it, its value goes down.
This is why oil is priced in US Dollars; this creates demand for dollars and gives the US government power. This is also why the Brtish pound tanked recently; people didn't like the direction of the newly elected British government, and sought to sell their GBP denominated assets.
So if your definition of ponzi scheme is "people trying to get more people to adopt it because it enriches them", then yes, all currency systems are "ponzis", they compete and have incentive to take value from other currencies.