The unironic portrayal started ramping up during its decline (4th to 6th edition I suppose?) as it was losing market share to other miniature products like Warmachine, Malifaux and Infinity. In reference to 40k, I think the unironic portrayal of the Imperium is being reversed a bit. I know, I had to read mountains of that garbage when that Google douchebag sent that memo arguing against gender diversity. You don't have many on Resetera, because they get banned, but the type of player who rollplays Paradox 4X games are often the same type who try and justify BIOTRUTHS and the genetic inferiority of black people with 1,500 word forum posts.
That isn't anything new but these sort of wargames have a whole lot of number crunching involved and tend to attract those who are naturally neurotic and pedantic. If you're a huge fascist or tankie, you're going to get your rocks off playing as Nazi Germany and steamroll Russia or as Soviet Russia and steamroll Germany. It has to do with the whole roleplaying aspect of these games.