Yes, it is if you are using the raw pack method. Sometimes, a few weeks after canning, peaches turn a shade browner. Personally, it doesn't bother me. They are still good to eat and are perfectly fine. Some people like to add a little bit of lemon juice to the jars to prevent that. If you decide to do that, use bottled lemon juice and not fresh lemon juice since its acidity is uniform and known. I've never done it so I can't tell you for certain that it works.  If the change in color bothers you, you should hot pack your peaches. Cooking them before packing the jars prevents them from turning brown.