The opposite is true.  Mastodon network is growing (new servers popping up) and nostr network is shrinking, due to neglect.

Nostr relays do NOT popup when needed.  They just die as they are not supported.  And we move ever closer to a model of a few central relays.

Mastodon's problem is they are tied too much to one domain.  They can move domains but it's hard.  What they could to do is add a nostr key to their profile.  Just like Nostr added NIP-05 to their profiles.  Then users got best of BOTH worlds.

#ditto is doing this.