There's at least three ways you could ask the question.

There's ontologically, in the manner of pedants, where you're really asking "in this defined ontology of financial phenomena, schemes and frauds, where does Bitcoin fit in?"

There's descriptively, where you are asking if it is useful to model or think of Bitcoin as a Ponzi scheme, regardless of whether it is a fully accurate categorization.

And there's rhetorically, where you are really just using "Ponzi scheme" as a concise way to tell people, "Don't invest in it, people are trying to take advantage of you."