As Newton's Apple said, it could be a focus issue, but I'm also looking at this thinking it looks like some pretty extreme noise reduction was applied to this image. Was this is really low light? You can see the painterly effect throughout the image, but especially on the fur towards the tail, where all detail has been smudged away. This is not unusual with extreme noise reduction. Can you post examples taken in better light? If you still have your 6s, take the same photo in the same lighting conditions with both cameras.When you are shooting indoors, there can be a huge difference between a shot taken one foot from a lamp, vs one taken 5 feet from a lamp or light source.
Update: EXIF shows this was only ISO 100, but at 1/4s shutter speed. That's very slow, but this doesn't look like camera shake as much as it looks like noise reduction. Was the original underexposed and then you edited it to increase brightness? That will result in loss of detail as well. Overall I'm pretty impressed with what the iPhone X can do in low light, but sometimes the auto settings don't make the best decisions for shutter speed vs ISO, or don't set the ideal exposure. Manual camera apps can help in that regard (especially those that support RAW) but I've found the native camera app gets it right more often than wrong.