If you’re a creator who still primarily or exclusively posts to Twitter after Nitter was killed off, you’re not gonna have an audience outside of Twitter anymore moving forward. Meanwhile Mastodon will let people see or even interact with your content without a Mastodon account as anyone can see a public post without an account, period, and anyone with an account on any ActivityPub or ActivityPub-compatible platform can interact with Mastodon posts.
If you want to post your own content on a public instance, you do need an account for that, though, or you can self-host.
But basically, if you’re still posting exclusively to Twitter, you won’t have an audience outside of Twitter anymore, while someone on PIxelfed could interact with a Mastodon post or vice-versa, for example.