Is the evaluation section of a paper a "contribution," if it only describes the performance of the proposed system with no novel ideas?
There are no official standards for what constitutes a valid use of the term "contribution".  It's intrinsically a vague word, and authors can use it in many ways.  It would be inappropriate to list something as a research contribution if there's absolutely nothing at all original about it, but a small amount of originality could be enough (and opinions can differ as to where the threshold is).  Of course it would be foolish to highlight the less original parts of a paper as contributions when this might distract readers from the more original parts, but that's a different issue.

You haven't given enough details to say much about your particular case, but making small modifications to an evaluation framework could be considered a small contribution.