You have many assumptions throughout this post, and others, that need to be put aside to properly consider the question.
There is no 'good' genetic material. All genentic material is of equal weight.1. Random Mutuation allows traits to appear that may be beneficial to the species. If that RANDOM mutation actually is beneficial to the species and it gets passed on, then natural selection can occur.
It could be that 'naturally very skinny' - assuming this trait is genetic - does provide a benefit to the species. If it does, your offspring who have inherited this trait may be more prodigious, genetically, several generations hence, than those offspring that did not inherit this trait.
It seems you are interpreting that 'naturally very skinny' on a good or bad continuum. This is a false categorization based on your subjective view of genetics and evolution. You need to discard that good / bad paradigm, and refocus on the hypothesis.
2.The assumption that 'survival of the fittest' requires that you 'fight' for survival. Nature provides ways for species that are unable to 'fight' for survival to continue their existance.3. Off the top of my head, what about fish? the female lays eggs in the water - the mail adds milt to the water (fish don't have intercourse) - what keeps the species going when these eggs - fertilized and unfertilized are tasty treats for other fish in the ecosystem? The answer is the number of eggs. Nature allows fish to create thousands of eggs during the spawn.. Many of those eggs will hatch (some wont), many of those fry will grow (some wont), some of the parr will mature (some wont), eventually, the offspring may get to spawn (many wont). With fish, there is no fighting, as you describe it, is irrelevant. It just a matter of luck, and numbers.
We are a clever species. But is being able to examine micro biological life forms as clever as, oh, say, polar bears having white fur, fish swimming in large schools, or chameleons being able to change the pigment of their skin? I do not think so. Each of those traits are part of nature, and cannot be separated from nature. I see human existance in a similar view.
Everything we do, from filling swamps to make airport runways, (even though it is destructive to the ecosystem) to genetically modifying corn so that it is more insect resistant, is a part of nature, not apart from nature.
In some Eastern philosophies, they claim that we are all one. Is it so difficult to believe?