You really think Nick was the cause of Mabel's mental issues?
Take the title for instance. Mabel may have drank occasionally, but she wasn't shown to be constantly under the influence of drugs or alcohol. It seems the "influence" in the title is Nick (with possibly the rest of Mabel's family & in-laws as contributing influences).
The beauty of the film is that trying to figure it out is left up to the audience. And when it comes to mental illness, not even doctors can always lay their finger on a single causative factor, so no one can say decisively. My impression is that the film was saying that MAYBE in a different situation or relationship, Mabel may have been okay (maybe occasionally neurotic, or flighty, or flakey, but not driven insane).
She's only home with Nick for an hour or so after returning from the hospital before she tries to slit her wrist. (Now did she have suicide attempts in the hospital? We'll never know. But if she'd had any recently, they probably would not have released her.)
But I think it's pretty obvious that Nick has severe issues (different from Mabel's, but in some ways far more destructive to those around him).
Mabel may have been prone to problems organically or through her upbringing, but we don't know. I think it's fairly obvious that the film is saying that, whether prone or not, Mabel's family situation & violent, extremely volatile, emotionally out-of-control husband was a major catalyst in her break-down, which may have built up for years until it reached a critical point.