This competitive Overwatch guide covers the advantages and disadvantages of various skins and cosmetics for different Overwatch heroes.
The differences in skins can vary, from point-of-view differences to color patterns that may enable (or hinder) enemy aim. This resource contains tips and tricks for evaluating skins on your own — so you can evaluate new skins as they are released. In addition, there is a near-comprehensive list of skins on a per-character basis, noting which skins may be advantageous and which may be disadvantageous.
While skin choices are certainly not the most important aspect to improving your Overwatch gameplay… they can grant very minor advantages and disadvantages sometimes. It may be worthwhile optimizing your cosmetics choices. That said, it’s important to keep in mind that other areas of practice are a much better place to spend time.