Since it is implied by the devs that the darkness is natural, does that mean that megabird is evil for trying to drive it out? Alternatively if Megabird is good, what if alef wasn’t completely a being of light, but also of darkness?
Eh, good and evil are a bit too subjective to apply to natural forces like this. If anything, we could argue that both sides are simultaneously good and evil, depending on perspective. For example, poison ivy is natural, but you don't want it touching you, because it'll hurt you if it does. From that perspective, poison ivy could be called evil. Does that make it inherently so? Not necessarily
Near as I can tell, darkness is like that for light. Is it natural? Yes. Is it "good" from the perspective of light? No, because darkness is the antithesis of light and, as we see in-game, is actively harmful to those connected to light. But that does not make it inherently evil, either
It's worth noting that, within the Sky Kingdom, very few things of pure darkness exist, as opposed to the vast abundance of light entities. For darkness, we have only the dark plants (which could themselves be corrupted variants of a "neutral" species), dark matter (arguably the only "pure" form of darkness we ever see), and the dark water (which seems to be polluted by a sludge-like substance). For light, we have all the kinds of light creatures, the spirits and elders (and Megabird), the light clouds and geysers, and our sky kids. Everything else is what I consider "neutral," as in, it possesses qualities of neither side. The peaceful crabs in Sanctuary, as well as the koi seen at Home and up by the Prairie temple, would be considered neutral entities
Darkness has corrupted some entities: the vast majority of crabs, as well as the light dragons, but light dragons were, as the name states, light creatures at one point. Could there have been "true" dark creatures at some point? Possibly! That may very well be what the massive skeletons in the Golden Wasteland were. There are also the strange creatures we see in the Switch trailer, which are almost like crabs, but not quite. These actively flee from the light released by the spirit's lantern, and we haven't seen a trace of them since
It's possible that the majority of dark creatures/darkness in general are found within the depths of the ocean. Perhaps the waters rose after the Fall and consumed much of the Sky Kingdom's lower landmasses... or perhaps they were pulled under by something. It would make sense: light, as it is connected to heat/flame, rises upwards to the heavens, and darkness, in the form of crabs, dark plants, and the gaze of dark dragons, is focused on the ground, moving downwards
There's also the parallel to the idea that light is love and connection, and therefore emotionally (and literally) uplifting, while darkness is isolation and a lack of love, and therefore keeps you low, both emotionally and literally (can't fly when you have no light, after all). It would explain the aggressiveness of corrupted creatures: being lonely, especially for long periods of time, leads to a great deal of stress and puts you in a long-term "fight or flight" status. In the case of crabs, they recognize that our calls can flip them over (and possibly see us as predators, given the roasting spit in Wasteland and Pleaful Parent's memories), and so preemptively strike (the "fight" aspect); in the case of those creatures from the trailer, they run away from the light (the "flight" aspect)
Similarly, the isolation of darkness seems to be tied to sound and hearing, as well. Our calls, as well as those of spirits and light creatures, have audial and visual components. When the bubbles of light come into contact with other light entities, they react (sky kids can be recharged, and light creatures will call back). But dark dragons don't react to these sound bubbles, despite being (former) light creatures. They don't react to fireworks, instruments, the cries of spirits or crabs, or even other dark dragons. As far as we can tell, they're completely deaf. This may be both a side effect and contributing factor to why they're now dark creatures—their deafness prevents them from connecting to others, which strengthens darkness' hold over them, thus creating an inescapable cycle
As for whether Resh is/was a being of both light and dark, it's hard to say. Light is almost definitely present. Darkness? I'd argue that no, not at first. If anything, the darkness came later in life, but it's hard to say exactly when