Blurry screenshot

Are your screenshots blurry after sending them to other people, using 3rd party apps, but they look perfect in your Photo app on the iPhone?

If yes, check the settings for the Low Image Mode in Settings/Messages, to make sure it is turned off.

To rule out a software issue, backup your iPhone and set it back to factory settings on your computer.

Set it up manually "as a new device" after that, without using any backup data or installing 3rd party apps.

Restore your iPhone, iPad, or iPod to factory settings - Apple Support

Then test the screenshot feature again. If it works, restore from the latest backup and do another test.

Do you have a zoom function enabled for your screen?

Are you using iCloud Photos and your screenshots are uploaded to iCloud?

Are other photos saved on your iPhone in normal resolution?

You're welcome 🙂

Good Luck!