I think it's mostly a symptom, yes. People in an un-good relationship look for things to take them out of it - phones are always available and don't have the same stigmas or costs as, say, always staying out late or lots of drinking[1]. Previously the blame often fell on TV, radio, news, etc, and now practically all of that combined is in your smartphone. It's really not surprising why people use them for distraction.

Are they causing more problems than stuff before? Probably! Much of what's on it is explicitly designed to be addictive. But I suspect it's by a relatively minor amount, and mostly stolen from other also-bad-for-you options that have declined / been out-competed. People have done this kind of stuff for literally forever, phones didn't invent it.

1: Not to imply they should or should not have that stigma. Just that they exist and work as a distraction too, and few people will immediately judge you for using it, unlike many also-popular alternatives.

Yeah, it seems fairly obvious to me that the underlying issue is that you're not being appropriately attentive to your partner. Whether that's because you're looking at a smartphone, addicted to meth, out of town on business constantly, working 20 hours a day on your new startup or whatever else is secondary.