Phones probably aren't helping, but there's another reason why mental health would be on the decline and willing participation in society is less than enthusiastic.
The free sharing of information on the internet has made it incredibly clear just how morally bankrupt our societies are across the board. You can't pretend that the suffering out there is miniscule when you can see it all play out over the internet. The world is a total mess, the people in power are largely malicious, we're headed for an environmental catastrophe of apocalyptic proportions, and we seem collectively determined to do absolutely nothing about any of it.
Why would people not be depressed? It's hardly surprising that suicide rates would be up in such an environment, nor is it surprising that some people cope with it by leaning into the horror hard and straight up endorsing it.
Like okay, maybe kids should use phones less, but what really needs to happen is humanity needs to develop some humanity. We are literal monsters complicit in mass torture and environmental destruction. We have some work to do if we want to not start feeling suicidal the moment we stop and look around.