I took this image with FairPhone 4, running LineageOS 22, using the default camera app.
https://obzorje.kompot.si/s/cPaYdRHQf2CG6iL
Tried playing around with some other camera apps from F-Droid, but all of them make pretty much same pixelated mess as the default.
No idea what the reason for this is. Is the camera sensor on phone actually decent, but then some algos in camera app fail to optimise things or something?
No idea, but I am quite often ashamed of sharing pictures with other people.

Luckily I am a kind of person who takes more "utility" type of pictures. Markings on devices and other kind of informative stuff.
If taking pictures of beautiful views and social events was important to me, I would probably not be using FairPhone 4. I would want to use it, but with the current state of camera on de-Googled phone, for artistic purposes, I could not.

Maybe its the e/os variant causing the complaints? (which is an opencamera fork)

Using e/os Fairphone 4/5, you can enable the fairphone camera by going to Settings:Apps:Enable Fairphone Camera and then restart the phone.

To further your point, I actually think the fairphone 6 photos look better than the iphone pro max 16:

https://www.youtube.com/watch?v=7oL6csRQYJ4

The more light the better the image will look. The reason others complain and darker images look so bad is because modern phones and apps use different techs to stitch together what the app thinks is a good photo.

The images I've seen from fairphones (as in the person posting included that info), the only problem I've noticed is when zooming. Tends to not have as good definition as other phones in a similar price range.

But the colors and overall contrast have always been solid, and not ever image needs to be zoomed into, so I call it a win tbh. Considering the limitations of a phone camera in the first place, I'd say fairphone does rather well