Everyone that owns a camper shell could use a roof rack, for extra storage. However, NOT every camper shell can support a rack. Actually, very few are designed for this, and installing a rack on many caps can void the warranty. Many shells weigh well above 300 pounds and most of that weight is from the roof.
Unfortunately, many fiberglass camper shells take a beating just driving regular city roads, let alone off-road driving. The reasons come down to the flexing of the truck bed’s side walls, slope to the sides of the caps, the weight of the cap and the lack of support of large spans of fiberglass. Fiberglass flexes, so some companies build support into their product’s structure, like a skeleton, allowing for heavier loads to be carried. This drives up the cost, but helps to prevent that nasty cracking you’ll see on many shells near the upper right and left corners by the back glass. Adding a roof rack to the top of a cap with these cracks is a terrible idea, as the cap is already failing under just its own weight! Increasing the load on the roof ultimately increases the force that pushes your bed rails and the part of the camper shell that rest on them, away from each other.
There are a few companies that engineer internal support, which helps to stabilize the camper shell under increased loading. They’re heavy and expensive, but if you don’t mind spending the money and not removing your cap too often, they’re a great way to go. If you already own a camper shell and don’t have an extra $4k+ to spend, you can try to figure a way to transfer the load from the roof to the bed. Since mounting anything to a topper voids most manufacturer’s warranties and you’re still considering doing it, there are ways to help you get the most mileage out of your fiberglass.