Answer
Cheating and plastic surgery are cultural artefacts, genetics and looks are physical nature. You imply that the change of physical nature through culture was against human nature, that physical givenness (genetics, looks pre-surgery) is human nature. This is not the case. The change of their physical nature (natural givenness) is an expression of human nature.
In principle, this is governed by morals - like all human action. You may accept this or you may not - it really depends on who you ask and what your values are. Morals as a cultural reality (might be different for their basis) are arbitrary: They are dependent on their socio-cultural and historical place.
On the other hand, there is a line to be drawn apart from historically and culturally contingent morals: These changes through cultural means have to be lived by and in the corporeal existence that is their condition. Boundless change of physicality (plastic surgery, kybernetics, drugs) has a limit in the fact that the human existence is both physical and cultural (both in themselves and their surroundings):
Whatever humans do to themselves or their surroundings, it has to be endured by their physical body that they (also) are and the nature in which they are. The absolute limit is that where a) the body or the world cannot physically bear the change (often reported by women with massively enlarged breasts, toxical environment) and b) the subject cannot identify itself with the body it is (psychological dissociation*) or its surroundings (think members of uncontacted peoples in a modern metropolitan area like Tokyo) anymore.
*psychological dissociation between subject and body is immanent in torture and rape, see Bernstein, J. M. (2015). Torture and dignity: An essay on moral injury. University of Chicago Press.
(another aspect is sociality - acceptance by other persons that are a constitutive element of the socio-cultural existence of the human - but this would go to far for this answer)
Insofar, the gist of the question is not nonsensical.
On the other hand, your question is incoherent (something also pointed out in the answer of @MichaelK): On one hand, you base the question on the idea that there is a fundamental divide between the laws of a perpetual physical nature (evolution, genetics, natural looks) and cultural expressivity (plastic surgery), but at the same time you have to admit that plastic surgery is an interference into physical nature, rendering this in ethical - cultural - terms (cheating).
Ironically, you can do so exactly because of the twofoldedness of human nature you are questioning.
Philosophical background
This is an insight of the philosophy of Helmuth Plessner. For a very short introduction, you can look here.
One aspect of what he calls "excentric positionality" - the defining characteristic of the human - is their "natural artificiality": Humans are cultural beings by their very nature. They cannot live up to their existential conditions without forming themselves and their surroundings.
Excentric positionality means that the human is not setted, not "at home" in the mere physical existence of living in one's body in natural equilibrium with its surroundings.
This is something Hannah Arendt rendered in her book The Human Condition (1958) as the difference between labor - sustaining one's life functions - and work and action - changing the conditions of one's own living by creating physical things like tools and houses (work) or by speech and writing in the political sphere, changing the social conditions (action).
The human condition per Hannah Arendt is that it has to do all three in order to live up to that which is specifically human: Animals sustain their physical life as well, some of them even build physical homes (birds build nests) or use tools (apes, ravens - it is questionable if they produce tools, though), but only the human can have a political life and influence social conditions per cultural expressions (language) instead of physicality (animals define their social position through physical means).
Plessner is able to catch that (and much more) in terms of the existential structure of the human, though, and can explain why the building of a nest is different from the building of a house (beyond the scope of this answer).
Technical details of the philosophical background
To give a more technical description of natural artificiality based on Plessner's book The Levels of the Organic and the Human from 1928 (English translation by Millay Hyatt forthcoming):
The distance from the lived physical body [Körperleib], the not being setted in its own physical living, leaves the excentrically positioned being in a situation where it stands in opposition to the vital processes of interaction between its organism and its surroundings: it is driven into existentialism, has to ask how to live and what to do (ibid:309). In an excentrically positioned being, a distance between the subject of the act and the subject-core of the living thing is its constitutive principle (ibid:296–97): it has to actively make the vital life its own life because it is not “going through” the subject of the act simply by virtue of its positional character.
Animals are standing right in their vital processes, they are grounded in nature and carried by it, resting in it. The being of excentric positionality, on the other hand, does not have a complete “natural grounding” in that sense as only one half – the physical lived body – is grounded in nature: the grounding for the other half has to be established by itself so that it can live its life carried by what it established and nature (ibid:316–17). The balance of the whole that was provided by purely organic means for the plant and the animal (ibid:219) is lost because the whole of human existence is not inside the organism. In other words: the human has to “make himself into what he already is” (ibid:309) because of a “constitutive lack of balance “ (ibid:316, see also 321) immanent in excentric positionality.
As the whole of the human existence is not carried by the organism alone, it is only “natural” that the human tries to establish something in the world that corresponds to this not (already) being nature, a second nature or culture (ibid:310–11). And the only thing that can correspond to the nothingness of the nowhere/never of excentricity is something that is not positively given as such (ibid).
Only then, the human being can actively, spontaneously, as a subject determine the conditions in which the vital life has to fit and sets its life circle – as both excentric and corporeal being – into freedom by closing it in its excentric subject: humans perform their life as both excentric and corporeal, or lead their life as a whole [Ganzheit] – subject and physical lived body in equilibrium – only by living under conditions that they themselves formed (ibid:316).
In other words: Only if the requirements of the excentric positionality are satisfied, the excentrically positioned being can set itself into the subject-core of its corporeal existence and “be itself”, i.e. an “I” (ibid:392,325–26). Their form of organisation is excentric (ibid:193,307) and building itself a home is the only way the excentric life-form can live its organic life: It is artificial by nature.
At the same time, it cannot escape the corporeal bounds, has to live its life as the "I" that it is, which is setted in its corporeal existence it has become. Without identifying with one's corporeal existence, life cannot be lived. And the end of the corporeal existence is the end of life.
