FaceTime background is blurry on iPhone 13.

You may Blur the background with Portrait mode

On supported models, you can turn on Portrait mode, which automatically blurs the background and puts the visual focus on you, the same way Portrait mode does in the Camera app.

1. When you’re on a FaceTime call, tap your tile.
2. Tap  in your tile.
3. To turn Portrait mode off, tap the button again.
4. 

You can also turn on Portrait mode in Control Center. Open Control Center, then tap Video Effects.