I disagree, Jim, I think you're missing the point of Duke.
His jokes are supposed to be shit, but he's still not supposed to be called out for it or portrayed as unfunny or less cool because of it. He's meant to still be idolised in his world, which makes the joke funny for us outsiders looking in. We don't idolise Duke, we laugh at him and the morons in his universe for still idolising him, and get to feel kinda badass that that's our character.
We go into the game already knowing that he's a total doofus. If the game took the time to tell us this, it would lose some of its impact. We just accept the world as silly and fun, and love it for that.
Also, just because Sam Stone was made out clearly to be an idiot, doesn't mean Duke has to be. I'm all for games varying from one another, otherwise they bleed into a one flavour formula that never does anything different.
In an age where "realism" is becoming more and more predominant, some games just need to get more and more outrageous to be pure fun. I don't play games to see my character do something I can do in real life, I wanna see him curbstomp,punch through heads, and have a three-way with a school twins.
And I've never played Devil May Cry, but it seems like a whole nother kettle of fish. Sounds like a serious drama, where Duke is a light-hearted comedy. You're not supposed to read into anything too much.