This assumes anything would evolve in such an inhospitable environment. If it's cold and oxygen is thin, it'd likely be as barren as mountain tops.

I'd suspect they would perhaps be a popular place for large flying creatures who might like to make nests there in safety and use them as resting locations during long migrations

They'd probably also attract humans and their ilk who wish to capitalize on the islands' natural defenses as well. Though this would obviously come at a cost of seclusion. Goods would be difficult to transport and would probably cost more than in a city on the surface.

I'd imagine they might still find settling up there worthwhile to mine whatever unique materials these islands hold. Perhaps the islands are composed of rare magical minerals that cause them to float. Minerals that would probably fetch a high price.