Update:

While "free" it seems that Mammoth [edit: encourages] users to register on their instance moth.social. They present this as "simplifying" sign-up, but some counter this is antithetical to the decentralized nature of the fediverse.

While currently "free" at some point this will change to a subscription model: "Therefore, we likely will have a subscription, but it won’t stop you from enjoying the app if you’re not paying. We don’t have all the details or the timing figured out yet.”