The problem is that it lost its sense of meaning: if everything in the movie feels completely fake and over-the-top, then I cannot care about what's supposedly being depicted at all.
It is hard for movies that try to be experimental to achieve this balance between experimentation and suspense of disbelief. Kubrick's 2001, for example, has a lot of experimentation, especially for a science fiction movie, but it managed to maintain its experimentation consistent with the movie as a whole.
At what point does a film need to adhere to realism, or need to be believed, to retain meaning? Sometimes it's more about the techniques a director uses where we find the core value in a film. Not in its ability to perpetuate an illusion of real character living a real life.
Absurdity, the grotesque, camp, surrealism, satire, farce are just a few approaches a director can use to step away from what can be passively accepted as being real, and force us to contemplate the artificiality of cinema. All of these things can still have meaning though, even if they push our suspension of belief well past the breaking point. Sometimes the absurdity is exactly the point.
Personally, I think someone like David Lynch and his approach to filmmaking has more to say about real world America than most sober minded directors, and his entire approach is about the embrace of the artificial. He makes films that make sense on a primal or subconscious level, that we hopefully also can understand intuitively. That short circuit reason to get to more abstract truths. Thankfully, not all films take this route, but equally thankfully, some do.
It all gets down to what we are looking for in a movie. For some, to enter into the world of film is to simulate a dream state. And as we all know, when in a dream, we can accept the most preposterous of scenarios as reality. In fact, they sometimes say more about reality than reality does. Just last night I dreamt that Donald Trump had his head shaved, was stuffed down a sewer pipe and had his face attacked by rats. And as I stood there in my dream, looking at the former president bleeding from his cheeks, his suit dripping in toilet water, his shorn head wet and shiny and grumbling about unfair the world has been to him, while it is a preposterous image to think of once I awoke, in the moment of dreaming it, it has more to say to me about modern politics than anything I've read recently in a newspaper. Sometimes what our rational mind rejects is brimming full of tactile or emotional or some kind of subconscious meaning. Sometimes being too careful about being realistic will destroy this type of illusion.
I think this more surreal approach is what mother is going for. I think we are meant to relate to the basic nightmare scenario of one unwanted guest after another showing up at our home. Thats all we should need to remain on board as it proceeds to have less and less relationship to what we can accept as reality and moves more and more to the sort of thing we might remember from a nightmare. The films hysteria is an outgrowth of all the anxiety and apocalyptic terror that is unfolding. To dial it back may diminish its impact.
Now that doesn't mean we have to like this particular example, or that there aren't criticisms to be made. Like I said, I'm not sure how well it worked for me either. But I still do think we can find an emotional or spiritual or intellectual or allegorical meaning in a piece of art, even if it refuses to play in the sam sandbox of reality we normally live in. Sometimes I find it to be even more honest than a film that demands we believe it as some kind of real world truth