A Woman Under The Influence (TCM Tonight!)

I think that's what the film was trying to convey.
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.

I think if it was anyone except Mr. Jensen, it might not have mattered, but he was the opposite of her - very stiff, didn't smile until the kids put on a show. Mabel has child-like qualities, wanted the kids to dress up and do a show, but I'm rewinding in my head, and based on that scene alone, I don't know if she acting that nutty, but when I become an uncle soon I wouldn't leave him or her with anyone! Also, Maria took off her clothes right before Nick came in, and for someone observing (like us) it would seem odd, also considering a stranger is in their bedroom. And yeah, Mr. Jensen could have called the police, but I'm sure he thought Nick was just venting.

I also think guilt played a part in the mother's change, but it's still surprising, for her to be the voice of reason. And regardless of what she says, and how different she changed, her son Nick always listens. I'm guessing Nick's father wasn't around (or dead), and that there's no manual for being a parent, so of course he's going to listen to his mother, or at least lean on her.

I think Nick turned Mabel from eccentric to a little nutty. We can tell early on that it doesn't seem like they ever have time together, and she's a bit nervous letting her mother (real mom who can't drive) take the kids. She even mumbles to herself if it's a good idea. She was running back and forth with bikes, and obviously concerned about her kids, even though they were with her mother. I loved the conversation as she's pulling out. I'll paraphrase, but to the extent of grandma not calling because she doesn't wanna bother, and then knowing this, Mabel will constantly be thinking is something wrong because Grandma doesn't want to ruin Mabel and Nick's special night, since it seems like it's been a long time (also talked about between Nick and his close co-workers). I also notice how Nick's mother isn't close with the children at all, and I don't remember her even talking to them, where they seemed comfortable with Lady Rowlands (Gena's mother).

Definitely not a comedy, but there are a few amusing and funny parts. And I'm not even referring to the "You've got a big ass!" which I thought wasn't something to envy back then. Not a very important part, but I know others who think that was funny. Next time I watch I will take notes.

You really think Nick was the cause of Mabel's mental issues?

If not a cause, then a catalyst or major exacerbating factor.

I thought I heard Cassavetes say that Mabel wasn't crazy.. There's a great audio of John and Gena at a University. It might be on youtube, if I find it, I'll post it on here later.