The addiction being a mental illness thing is an oversimplification. In some cases it's the cause, yeah, but there are other contributing factors, self-destructive people in general, thrill-seekers who engage in it too long and go down the rabbit hole, gateway drugs and longtime bad habits getting out of control, hell, even people that just genuinely enjoy it and don't want to get out of it. There are plenty of germinating factors in someone starting to abuse substances, ask anyone who works in rehab. One-size-fits-all mentalities, dangerous they be, says Yoda.
Your thing about Tobias and his hatred of his own reality all hinges on whether he intended to get trapped. That's not even close to clear. He knew the risks and consequences, grasped them as well as any of the other kids. He was just the one who had to actually deal with them outside of the theoretical, the potential outcome became his daily life. Irresponsibility? Sure. He's a 13 year old kid. Making smart decisions isn't exactly what teenagers are known for. Mental illness, though? Incapability of grasping reality, not able to comprehend or manage risk and consequences? Grasping at straws, big-time.
Poor coping skills is...ehh, too. He adapted pretty damn well on the whole to both their new reality in terms of now being in a war, everything changing dramatically, and in general terms to being stuck 24/7 as a hawk. As well as anyone could reasonably be expected to. There's a difference between Tobias' home-life being extremely s**tty and him somehow being either distanced from or unable to accept reality.
As for the responsibility thing, of course it was his responsibility. But he was 13, and 13 year olds screw up from time to time. About minor things, about major things. Completely mentally healthy ones. All the time. People seem to automatically equate the guy being unpopular at school and having an awful version of what passes for a family as him being somehow "troubled". Neither of those things are exactly his fault, and once the group gets to know each other halfway-decently by a few books in, he's one of the more social and amiable of the whole crew. Probably second only to Cassie for god's sake.
And, hey, he's not even particularly one of my favorite characters of the group. This sort of assessment of the guy's pretty out-there, though, unsubstantiated conclusions are pretty much the major factor in what's wrong with the world right now.