You can certainly find an opinion somewhat like that in Francis Galton (Darwin's cousin) -- and, truth be told, here and there in Darwin himself, too. (Biologists don't like to admit that, because the great man has been rather hagiographied ... but it's a fact.) However, you immediately run into a problem with evolutionary theory if say that it's everywhere and always operative and always does the needful ... but at the same time needs a helping hand. Huh? How could it?
The brilliant Australian philosopher David Stove is very good on this:
Amazon.com: Darwinian Fairytales: Selfish Genes, Errors of Heredity and Other Fables of Evolution (9781594032004): David Stove: Books
I think the fact of the matter is that evolution works on such a long timescale, that all this was sorted out long ago. Human beings are very finely-tuned creatures well-suited to their environment and, while genetics is part of the picture, the heredity of Wal-Mart couple really isn't that bad.
What, I think, matters is epigenetics. A human being, whatever his or her genetic potential, is not going to realize it unless getting the ideal environmental circumstances (including, of course, nutrition) from pre-conception onwards.
If people get bad enough nutrition, then I suppose they won't be able to breed -- just like Pottenger's cats after a few generations.