That's inaccurate, a core feature of Ponzi schemes is that they require fraudulent activity, where someone is lying to participants about how much money is "theirs" and the way in which it grows.

Cryptocurrencies don't do that, the system isn't lying to you about how many whatevercoins you own.

Pyramid-scheme? Maybe. Pump-and-dump? Happens sometimes. Hugely speculative asset? Definitely.