First, I take issue with the idea of calling something "dumb" for trying to avoid modern app bloat, even as marketing. I understand the precedent of a "smart" device being something that backhauls your personal information to the surveillance industry (decidedly not smart!). But still, can we find a better name for the concept? "Native" ? "Lean" ? It shouldn't be about giving up some purported betterness of bloaty so…
I read it as in "dumb terminal". It's just rendering HTML as is, not trying to do anything clever. But maybe I'm just pining for the days of VT100.
My ideal client would be an Emacs one, focused on plain text but can still display images. No doubt this exists.
But.. I've never gotten into the Fediverse or indeed any social media. The decentralized, small web premise sounds nice though.