I think the problem with this thread is that there is a potentially serious topic hosted by one that has had a history of perversing certain topics.
To answer your question, there is no doubt that Angron became retarded. People have to understand that most people who are retarded are not born that way or at least the retarded genes are not developed until a few years into their upbrining. People can become retarded due to external factors, like accidents, drugs, etc... Angron was clearly made retarded because of the Butcher's nails destroying his brain functions.
The retardation... I think is most likely ignored when describing them because its not what fantasy players want to call one of the coolest chaos legions and warriors in lore and gaming. So thats understandable. The Butcher's nails are the cause, however, it stops there in the mentality of most gamers.
Its also interesting to note the differences between Bezerkers (Warriors with the Butcher's Nails) and other warriors and daemons of Khorne. There is a difference when we read descriptions of Khorne Daemons compared to those of the Bezerkers. In fact, that might be the reason why the mark of khorne for gaming purposes has different benefits than bezerkers. The simple, "they are warriors of Khorne, what do you expect," is not necessarly true.
For lore purposes, it doesn't sit well with me how they developed Angron. Before it was an angry Primarch with the need of vengence. Though some of the qualities still inherit Angron today, it really stems from the Butcher's Nails. Primarchs and Astartes are suppose to be vastly superior both physically and intellectually compared to average Joe. The fact that the World Eaters, were so flawed and tactically very weak, undermines this truth. We have astartes that can memorize thousands of pages of technique from the Guilliman's Codex, to a bunch of brutes charging heavy weaponry probably knowing they are going to die.