FWIW, I feel like I at least mostly understand the premise of Mastadon and how it works--though there are crucial details I know I don't related to how replies

FWIW, I feel like I at least mostly understand the premise of Mastadon and how it works--though there are crucial details I know I don't related to how replies interact with federation (exactly where the data gets cached and stored in such cases... it isn't clear to me if that content is "owned" by the server of the parent or the child, and then how other servers manage to find it)--and I'd still be extremely frustrated by a public/discoverable mirror/proxy of my content, and will claim that that consequence isn't just an "impression" but is actually a fair description of what is going on. Let's put it this way: is it really any fundamentally different from someone setting up an Instagram account for someone else who is only on TikTok and then copying all of their content there, because they prefer using the Instagram client or don't want to install the TikTok app? I'll claim "no": the federated nature of Mastadon doesn't actually change why people want to do this and it doesn't even change the mechanism for how it is being done. Now, maybe you are OK with such mirrors--and, FWIW, I can begrudgingly defend that position if required to--but I hope it provides the right framing. I'm also going to assert that you should not ascribe benign intent to people doing this, even if initially it seems fair to do so or you yourself are the person doing it and you swear you are just providing access: I've seen people do this kind of thing--including to me, as I have in the past been considered to have extremely high-value / sought-after content (back when I was posting updates about iOS jailbreaks)--many times over the years, and eventually they realize they now have a ton of followers on "their" account and can start inserting their own content or even modifying mine. Meanwhile, people end up trying to contact me by messaging a mirror account--as while some of those in your screenshot are ostentatiously adding "[UNOFFICIAL MIRROR]" to every tweet, not all of them are--as they absolutely do just look like an official account and so people follow it and then start responding and communicating with it... that simply never goes well for anyone: I have definitely had people reach out to me complaining about interactions they had with "me" when they were actually communicating with some random person providing a content mirror / translation. If you want to do something like this for a service, it should either be part of your client (as opposed to a server/protocol issue) or (barring that, and given Twitter now trying to red team people with their API, I get it) be more like how people build XMPP bridges: something that is pretty ostentatiously a special bridge mechanism to the consumer who probably themselves actively installed and configured it so it is really clear what is going on, rather than it merely being a bunch of "this is just a regular user account" accounts created by people who are claiming to be neutral.