Yes, this principle should apply in nature; but at the same time this only applies to a fairly restrictive case

namely, if you take a population of organisms and put it in a new environment, at first its adaptation to this environment will improve very quickly, then it will keep making progress but slower

however, in nature you have to consider that (1) most organisms have been in their usual environments for a while so they're all in the slow adaptation phase; yet that (2) environments actually change all the time, because climate isn't perfectly stable and because the surrounding ecosystems aren't stable either (ecosystems are complex systems that are constantly disrupted by new pathogens, new species, species going extinct or becoming less/more abundant, etc., on top of natural phenomena)

So there's always adaptation, because it occurs towards a forever-moving target. In most cases, it's more accurate to consider that evolution is a perpetual thing