If you’re using iCloud Photos, you might have the “Optimize Storage” option turned on. This saves your photos on iCloud and only keeps lower-resolution versions on your iPhone. To solve this, open Settings and go to “[Your name]” > “iCloud” > “Photos.” Tap “Download and Keep Originals.”
If you haven’t optimized your photo storage before, you can fix blurry images using the editing tool in the Photos app. Tap “Edit” and sharpen the photo using the “Adjust” tool.