Mastadon depends a lot on what instance you're on. You're community is primarily your instance and non-instance users are secondary (discovery is bad outside the instance). Unlike Twitter you have to be intentional about your community.

I've been on Mastadon since before Musk said anything about buying Twitter. I never viewed it as a Twitter replacement, but I never spent much time on Twitter to begin with.

I think a lot of people came to Mastadon to look for "Twitter but not Twitter". That's never going to exist. Not in decentralized form, not otherwise.