There's a lot of that. A ton of FOSS software is somewhat exclusionary because it's made for the people who make it.
But a lot of the UX issues on Mastodon have nothing to do with the tech, nor the UI. They're social in nature.The existing userbase skews technical, which affects what people discuss, and people looking for help are met with a deluge of tech savy people giving tech savy advice.
Oh, and there's the mass of very vocal users on niche sites that have strong feelings about having their niche safe space invaded by "normies", and who let it be known that new users should learn and adhere to "the rules" and respect the unlisted, unagreed upon nettiquite of social outcast "progressive" fedi or GTFO.
And then, on top of the social, there's just the fact that most Internet users don't really grok the Internet these days. Twitter or BlueSky aren't websites to them/ they're "apps". The very nature of federation on the Fediverse runs counter to how they understand how thir "apps" work.
They don't want to have to know about it, but they can't avoid people talking about it, making judgements around it, and having to confront it when edge cases crop up or when admins decide they don't like or trust the new crop of fedi websites that have sprung up this month or last.
On Twiiter or BlueSky, they don't have to think about any of it.
ETA: Things might be different if people stopped treating "Mastodon" as a place that exists on the Internet, but even the Mastodon developer treats it that way, when it's convenient to him. He's created a little functional monopoly, and seems to care moee about that than anything.
Mastodon servers are Mastodon branded, and that is a mistake, in the long run. We need to communicate to people that they can sign up for MyInterest.social, that is MyInterest branded, while also getting to follow people elsewhere. That overcomes the biggest hurdle.
But that doesn't satisfy the egos of people in positions to right the ship.