I'm going to address the general concept of stuff around desks, even though your question seems to be narrowly asking about stuff on the wall. If that matters, then it would matter if the wall was painted white or if it had a busy wallpaper pattern. That sounds preposterous magical-thinking-like to think that wall coverings have much impact on anything.
As for stuff on the desk, this is personality dependent. Everyone is different about how much clutter they are comfortable with. Personally, I don't notice clutter on the desk. I clean up the desk when I can't find something. For me, organizing and choosing a nice place for everything on the desk is premature optimization.
On the otherhand, if the clutter on the desk causes psychic distress, then I guess you don't have much choice to do something about it. Cat vomit on the desk would cause me psychic distress until I cleaned it up.
On the otherhand, putting 30 minutes into tidying up a desk everyday would be a time drain with no return on investment. At a half hour a day, hypothetical desk cleaning would take 130 hours a year and there is no way that being able to find a lost time 10 minutes faster would offset that 130 hours for me.
On yet the other hand, at a dentists office, needing to find a patients records among the 1000s of records is going to happen ever few minutes, so spending 30 minutes a day to tidy up may actually pay back more than 130 hours of saved time looking for stray patient records.