Yes, but phones are not those things that are okay, unless they're very well-manicured.
It's not mindless, it's far more insidious than that. There are people vying for your attention. They are not doing it mindlessly, and they are not doing it for your benefit.
Almost every mindless activity mankind has ever known hasn't required a phone-sized computer with a touch screen, but somehow that's nearly the only one people are doing anymore. I think that speaks to the effectiveness of the algorithms.
But to your point: Amen. Spend some time getting bored. It fixed a lot for me in the motivation department.
Today I think it's okay to not optimize everything for efficiency and it's also okay to waste some time on mindless things.