Most stomata are located on the bottom of leaves because direct sunlight does not hit the bottom surface. If stomata were on the upper epidermis (the top side of the leaf), the direct sunlight hitting the leaf would cause more water to be lost through the stomata because of evaporation. Hope this helps!
It depends on the plant and its environment. In a mesic terrestrial environment, the plant wouldn’t be optimized. Natural selection would then come into play and there would be an evolutionary trend towards stomata located on the lower leaf surface.
In the case of water lily, the lower epidermis is in contact of the water, and stomata are restricted to the upper surface of the leaf. This is an optimal solution for the plant. Natural selection will work to conserve that arrangement. In this case, stomata on the lower surface would exist at a water-air interface, and the diffusion of gasses across that interface would be glacially slow. Further, the stomata on the lower surface would present an opening where pathogens can enter the leaf.
