I would think that she's not only trying to cool off, but also to keep bugs/biting insects off. Or maybe trying to get enough attention so that you bring her inside
Mine roll in dust all the time. The vet says it is their way of staying cool, it also helps when they itch or have fleas. Even when mine are flea treated, they roll. I remember awhile ago watching the horses roll, and Bailey one of my kitties was nearby. Suddenly he dropped and started rolling too. Funniest thing I ever saw, but by the time I fetched my camera the animals were done with their dust bath.
My cats also like to roll in the dust. I have learned recently though, that dirt is one way for internal parasites to find their way into your pets body. Mice and other rodents, and even some insects, carry parasites. The parasite eggs can drop into the dirt by way of feces. The cat rolls around in the dirt picking up these parasite eggs, then she grooms herself ingesting the eggs. I would suggest that if your pet is outside, and especially loves being in the dirt, that you keep her well guarded against internal parasite activity.