I think StreetComplete (and before, this gallery) went the wrong way by stressing visual comparison of the situation on the ground with example photos. I think the way to tag smoothness is to try to imagine using the way with the vehicles mentioned in the wiki. See also here. The explanatory text and the example photos in the wiki were added by me only 3 years ago (with discussion only on the talk page of the wiki, as I hadn’t found this forum yet) to make tagging a bit less subjective and more verifiable. I proposed to move away from visual evaluation in StreetComplete here but it was rejected.
I know, it’s the reason why I picked this photo as the first to discuss because I wanted to be sure it’s not only me who thinks it’s not suitable for illustrating smoothness=bad: at first sight, the surface quality looks as good as the “mostly even” image, and only when looking closely you notice the wide gaps between the bricks. I think it appears about as rough as the “A little bumpy” picture.
I will, as soon as we have consensus here on what should be on the Wiki.
I have in mind that we discuss most of the pictures in the Gallery here, beginning with what seem to be the most controversial ones, and then move a picture if deemed necessary and add a caption with a summary of our discussion and the result of the vote. I think edge cases are especially interesting to show what different mappers are considering when tagging smoothness.