why do pictures appear low quality until I zoom in?

iOS 11 will enable "optimize storage" by default after the upgrade. Disable it again, if you have enough free storage on your iPhone. It is a pity that the iPhone will not warn you, before it starts removing photos from your iPhone.

See the iPhone user guide: http://help.apple.com/iphone/11/#/iph961b96c4d

Optimize your storage or keep all your photos and videos in full-resolution on iPhone. Optimize iPhone Storage is on by default. It manages space on your iPhone by automatically keeping your full-resolution photos and videos in iCloud and smaller versions optimized for iPhone, as needed. To keep the full-resolution originals on your iPhone, go to Settings > [your name] > iCloud > Photos, then select Download and Keep Originals. Your originals are always stored in iCloud.

Tysonh16 wrote:

I don’t have iCloud Photo Library turned on and it still does this. Would I have to turn it on and select keep originals??

No, you should not have to do that.

Why doesn’t it show full quality automatically with it turned off?

I don't know.

I don’t have iCloud Photo Library turned on and it still does this.

You have checked this in the settings for iCloud and for Photos & Camera, right? sometimes it gets accidentally turned on. Keep iCloud Photo Library turned off, if it is currently off and you do not have enough cloud storage.

Why doesn’t it show full quality automatically

If it is not an iCloud problem, I can only guess. Photos may still be reprocessing the adjustments after the upgrade to iOS 11 and be creating new thumbnails. Do you have enough free storage on your iPhone?

Do you by any chance use iCloud Photo Library with the "Optimize iPhone Storage" option enabled? Pictures that have been optimized for low storage use can behave like that.