Blurry iPhone X photos in "Portrait" mode

Make sure that the lenses are clean, make sure you are getting the camera the correct distance from the object when using Portrait mode, make sure you are holding the camera still and can see that the auto focus has completed. If that does not work, then try a Force Restart. Force restart iPhone - Apple Support

If that does not help, then you need to contact Apple Support.

Thanks to ASC User Lawrence Finch

Best Way

Install the Apple Support app on your iPhone or iPad - you can contact support directly from the app, and the tech will have (with your permission) access to diagnostic data on your phone

Other options:

• • Click on “Support” at the top or bottom of this page
  • Go to Contact Official Apple Support

(Note that both of the above have options to receive a callback or chat)

• • Call Apple Support; the phone numbers are in this link→Contact Apple for support and service - Apple Support
  • Make a Genius Bar appointment at an Apple Store→Genius Bar Reservation and Apple Support Options - Apple
  • Find an Apple store or Apple partner service provider→Find Locations

What part of the photo is blurry? You do understand how portrait mode works, correct? The part in the foreground, normally a person, is in focus and the rest of the photo is slightly out of focus. That is called bokeh effect.

I'm a little confused by your other statement, "the photo mode's display is good". Are you saying that before taking the photo and looking at the screen it looks good but the end product is blurry?