I think even when it came out, people recognised that most of the appeal was in things that shouldn't be appealing. Even to teens, shooting naked ladies was obviously an issue, which sorta seemed to be the point. I disagree that Duke was a character of his time, or a parody, except that senseless violence against the helpless seemed to be big at the time. Syndicate Wars let you set civilians on fire and blow up buildings for no reason, and Dungeon Keeper was about being the villain, which was relatively new and daring. Certainly, the crap catch phrases lean towards parody of action movies, but nobody seemed to care. People asked if the pirated version was the uncensored ones with the strippers that showed you their boobs that exploded, not so much about the bad lines.
OTOH, when the game came out, DOOM was still the game that FPS were compared to. Duke3D had major technical improvements over Doom, the levels were 3d in a way that Doom couldn't be...Doom just had a map with an elevation for each point on it, which meant you couldn't go under and over the same bridge, for example. Duke3d also let you swim and fly (the mechanic being identical), laser trip mines, guns that shrunk enemies or yourself, allowing access through small tunnels.
However, this seemed to be let down by the level design, which was pretty poor. Doom had been very inventive with what it had, but the levels were levels of a game to be played. The first level of Duke3d has you go to a seedy theatre, the one after has you go to an adult store/peepshow place, then to a club/strip joint. These might be more realistic than, say "Barrels of fun" from Doom2, but, well, "Barrels of fun" was fun. Shooting monsters and strippers in a strip club isn't much different from shooting monsters and strippers somewhere else.
Later on levels got better, but not all that much, IIRC.