Depends on the app, and the personal device. Mobile devices are increasingly thin clients. Of course hardware-wise they are fully capable personal computers, but ridiculous software-imposed limitations make that increasingly difficult.

"Thin" can be interpreted as relative, no?

I think it depends on if you see the browser for content or as a runtime environment.

Maybe it depends on the application architecture...? I.e., a compute-heavy WASM SPA at one end vs a server-rendered website.

Or is it an objective measure?

But that is what they are mostly used for.

Speak for yourself. Many people don't daily-drive anything more advanced than an iPad.

I mean, Chromebooks really aren't very far at all from thin clients. But even my monster ROG laptop when it's not gaming is mostly displaying the results of computation that happened elsewhere