Why we need eugenics
The reason humanity as a whole need eugenics is that we cannot survive on earth forever, eventually the sun will become so hot it will try earth and all life on earth will die, moving to mars (assuming that would even be sustainable at all) would just buy us time, we need to more to other star systems in order to continue our lineage and civilization.
Futhermore societies that implement eugenics successfully will be at a disadvantage over societies that do not make the population more fit. This however will not be the same as what's ideal for conolizing other planet. What's ideal in terms of genetics always depend on environment.
https://vintologi.com/posts/3484
There is of course a lot of different methods that can be used for eugenics, it does not have to be a policy that is officially justified with eugenics. One example of this is baby quotas supposedly implemented for raising the fertility rate but then it's individualized to specifically target privileged individuals (forcing them to have and raise more children). We can justify this by saying "privileged people need to do their fair share in increasing the fertility rate" and also "we shouldn't force poor people to have children" and also make exceptions for people with genetic disabilities.
If eugenics is generally viewed as something bad then governments will have to figure out ways to do eugenics without the masses realizing the real purpose is eugenics. A policy can officially have some other purpose such as "supporting parents" or "keep people safe from dangerous people".