It is filthy. Objectively, they've done studies on this and it's gross. That's not going to stop me from using my phone in the bathroom though.

But at this point I view someone's smartphone as a deeply personal object - I work retail and I often have customers ask me about something on the store website, I always ask before touching their smartphones. I don't know why, but it's always felt appropriate to ask. And then find the hand sanitizer after, because that's their germs, whereas my smartphone has my germs and I'm fine with that, lol.