The principle characteristic of a Ponzi scheme is that the value of whatever is being sold is hidden. For a publicly traded stock, bond, commodity, whatever, the price goes down when people sell it and goes up when people buy it. Everyone can watch that happen, and they know what their share of the pie is worth. This is why security exchanges exist, to track and publish those values.
With Ponzi schemes, early withdrawals are hidden, and the value of the security is artificially inflated. By the time a Ponzi scheme matures, the value of the security is far below the advertised value, and may even be negative.
With cryptocurrency, the value of what you have is publicly listed. There's no opportunity for that kind of fraud. Cryptocurrency CANNOT be a Ponzi scheme.
This isn't to say that you can't run a Ponzi scheme around cryptocurrency. If you invest in a fund that is supposed to be earning money through cryptocurrency arbitrage, you are probably caught in a Ponzi scheme. There is nothing about cryptocurrency that makes it immune to the fraud that people perform with any other form of currency. It just isn't as susceptible to arguments over ownership.