Frozen peaches or peas can actually be more nutritious than the fresh versions [source: Kern]. It's one of those rare areas where mass-production may be good for your health. But only...
because of the way the fresh-produce industry operates today. Fresh produce travels long distances to get to market, so it's often picked before it's ripe lest it go rotten along the way. Since produce develops its nutrients on the tree or vine or stalk, while it's ripening, interrupting that process also interrupts the development of all those vitamins and antioxidants.
Frozen produce, on the other hand, can be fully ripened before it's picked since it's getting frozen immediately afterward. There's no worry that it'll go bad before you can buy it. The end result is that frozen fruits and vegetables may be more nutritious than the often unripe stuff in the fresh section.
Fresh and ripe, however, is more nutritious than frozen.