Your definitions may vary, of course. For my purposes, a photographic style is just a set of photographic choices made in advance and applied consistently across a set of photographs, that creates a consistent look. Where to put the lights; how high the camera is set relative to the subject; what sort of contrast range are we shooting for in the final print. Too small a set of choices (for example, just a shutter speed) probably won't really make a style, since it will not create visual coherence across photographs. A good set of choices will determine a coherent visual look and thence a style.
The only thing that matters, really, is that an average viewer looking at the set should feel that the images fit together visually. They look similar enough in enough ways to feel "together" as a collection.
As with any photographic choice, or set of choices, a style either tends to support the images, or not. A visually dramatic style might well support a dramatic subject or collection of subjects. A softer style might support dreamlike images. However, a style not only supports or fails to support a set of images, but also may connect that set of images with other images we have seen. A style may refer to a single image, or an artist, or a fad. For instance, virtually all black and white landscapes made today are post-processed in a style intended to reference Ansel Adams, for better or for worse.
Pop art does many things. Viewed through the lens of this discussion of style we see that one of the things visual pop art does is to create a style by borrowing choices from popular imagery, and then to build art in that style. The style connects the work to the popular imagery. We might borrow certain photographic choices from visual art we see a great deal of, while rejecting other choices from the same art. Ideally, the set of choices we select will cohere into a style that not only supports the images we make, but also creates visual coherence across the work, and finally connects the work with the imagery we borrowed from.
We might, for example, create a portfolio of work identifiably connected with Facebook snapshots of drunk girls. We might borrow:
- on camera flash
- ... making "duckface"
- ... and throwing fake gang signs
Would it be good? Unknown, and it probably depends on a lot of factors. But there would be a style in play, and that style would certainly be working as a style. It would connect the work with the cliche we've borrowed from. It would connect the photographs in the portfolio together. Whether it supported the photographs well or no would probably depend on other factors.