...whereas a rope never spoils a rock climbing photo, a mat always ruins a bouldering shot...
Would an old fashioned beer towel be OK, its all we had in the past?
I don't know why, but understand your (il)logic! Clearly a top rope always ruins a shot though.
A rope in the wrong place can rui a photo, but generally they follow any lines in the rock and give the effect of leading lines.
I agree a bouldering matt (and chalk bucket, discarded clothes) does ruin a shot with the exception of a top down view, but it's pretty easy to frame your shot to avoid it.