Zack Snyder's Justice League in a 4:3 ratio, but, once again, this look is 100 percent intentional by Snyder.
Zack Snyder has always been known for his unique aesthetics, establishing a unique look that stands alone from 300 to Watchmen and of course his DCEU films. In the past, he's always collaborated with talented cinematographers like Larry Fong, Amir Mokri, and Fabian Wagner, but Army of the Dead marks the first time Snyder has been his own director of photography for a feature film, resulting in one of his most unique looks yet.
On a purely technical level, the reason the movie is so blurry is because the entire movie is shot wide open with vintage f/0.95 Canon Dream lens, giving it an incredibly shallow depth of field, soft bokeh, unique light artifacts, and potentially other aberrations depending on the condition of the glass. In fact, while most lenses have an adjustable aperture to change the depth of field and control for different light sensitivities, the custom rehousing Snyder did for his dream lenses didn't even have aperture blades built-in, meaning the entire movie is shot fully "wide open." In more plain English, he used a vintage lens designed for still photography in the 70s notorious for how little of an image it would have in focus at a given time, enabling it to make the objects in focus pop more drastically against a blurry background and/or foreground, while giving out of focus areas of the image a blurry dream-like quality, thus the name, the "Dream Lens."
Obviously, there are plenty of lenses that don't behave this way, so Snyder also had creative and practical reasons for choosing it. One of the explicit reasons he's mentioned, outside of the fact that he's a sort of vintage lense connoisseur, is the fact that this was his first time shooting on digital, so he wanted to shoot in a way that feels really organic, and the vintage lens is certainly one way to do that.
Many cinematography techniques are ultimately just a means of directing the viewers' eyes to particular parts of the screen, and there's not many more clear ways of doing that than making sure what you want the viewer to see is literally the only part in focus. Snyder also has an eye for iconography, particularly with the human body and striking "hero poses" or statuesque looks, and the way the Dream Lens can make the subject of a shot truly pop against the background and foreground compliments his approach a lot.
Lastly, this hasn't been mentioned by Snyder or anyone involved with reshooting Tig Notaro to replace Chris Delia, but in the end, it also aided in helping her fade into the background or foreground of shots that didn't primarily feature her, making that particular effect nearly seamless in most of her scenes, with one or two small exceptions.
Ultimately, while it is a defining aspect of Army of the Dead's aesthetic, it's not such a drastic effect that it's distracting or overpowering and there's only a few moments where it's utilized to create a stand-out shot. There are a lot of moments throughout where things go in and out of focus, but the focus always comes in sharp where and when it needs to. Even Snyder says he was worried the movie would be way more out of focus.
