Top layer is called the canopy. Your assumption about it being less curly because of exposure (not only sun, but wind and other elements, too) is correct.

I find that frequent haircuts help a lot. My fine/low porosity hair loves protein. I do at least one PT a week and use a lot of products with protein. You might find that helps a bit. Keeping the moisture/protein balance right helps, too. I use minute amounts of really rich products, in addition to moisturizing PTs, so don't need to DT.

My hair didn't find KCKT moisturizing enough. Using a protein and more moisturizing products might get the curl to match--but the other way around. Your canopy may become healthier and match the underneath!
3a (Corkicelli), highlighted, fine, low porosity

HGs: Anything Sevi; Curly Kinks Satin Roots, Curlycue ReNew and Coil Jam; homemade FSG and okra gel; soap bars; UFD Curly Magic; Botanical Spirits Jellies, CJ Repair Me, Aloe Fix