I don't know how to fully answer this question, but I think Mastadon is roughly equivalent to a Nostr client, where it is one option for interacting with Fediverse.
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A Mastodon instance is more like a Nostr relay except you have to sign up with an email and password, the admin chooses what other instances you're allowed to see posts from, and since your account exists only on that single relay, if it goes down your account disappears.
Not very decentralised and (by design) not censorship resistant because the creator of Mastodon loves censorship.
That's why the Nostr <> fediverse bridge was made by the Soapbox dev.
Makes sense about the instance but is it fair to compare Mastodon to clients in the sense that as a Fediverse user, I can use Mastadon or some other implementation? Maybe in this context, within the realm of ActivityPub software, there are multiple implementations to choose from that are interoperable with each other.
Yes that's fair to say for sure. Pleroma is also ActivityPub based but Pleroma users tend to be more chill while a lot of Mastodon users are Tumblr grade SJWs. As you can imagine many Mastodon admins ban Pleroma instances just because they use Pleroma.
But the big difference is that the client and relay are combined in the ActivityPub model regardless of what platform you use. You can cheaply self host Pleroma (Mastodon is very resource intensive) but running an open Nostr relay is as cheap and contributes more meaningfully to a decentralised protocol.
I've used both extensively (ActivityPub and Nostr) and by far Nostr is superior.