Do you have the "Live Photo" option turned on? Bad Low-Light photos have been linked to the Live Photo option. Make sure you turn that off by tapping the circle up top and making sure it's WHITE and not YELLOW.
Are you using the option to "Optimize iPhone Storage" in Settings > Photos & Camera? That makes it so when your phone is low on storage it will store low resolution versions of photos on the phone, and store the high resolution originals in iCloud. So if your storage is low and you're viewing your photos off the phone, and posting them here using your phone (which your signature suggests), then it's not the full resolution version.
But honestly it just looks like a badly taken photo. The iPhone camera is known as a "point-and-shoot" but in reality requires a bit more finesse, proper angle, and proper lighting to take great photos. The lens is the size of a pea and you seem to be expecting the quality you get on a dedicated camera 100x the size. You're not a professional photographer if you're using an iPhone so you shouldn't expect professional quality no matter what the marketing tells you. (You said you're not that stupid, so I'd hope you aren't expecting the quality of the example photos in the press releases)
Quick shots of your cat in bad lighting aren't going to look that great. But step outside in the natural sunlight and it takes amazing photos for the size of the lens and it also surpasses every other phone in low-light situations. In the photo you took it definitely needs more lighting like an open window or something. Anything better and you're going to need to invest in a nice SLR camera or start taking photography classes.