I think the only reason it's considered as such is because of the negative stereotype. We like Duke because he's all about ass-kickin' and takin' what he wants. Now, imagine if Duke had the same type of stereotype that the women have:
He'd be rail thin, wearing a douche-y shirt from American Eagle, with an insufferably nasally voice while he ran around making semi-racist jokes. He'd have glasses, and would be terribly out of shape.
In this case, they gave Duke a positive stereotype, while they gave the women a negative one. Unfortunately, they're missing the point altogether. The point being that this is a satirical game mocking the very thing it personifies.
I'm not offended by Duke. I am, however, offended by pretty much every other video game that has women in it ever, because they aren't -trying- to make fun of these stereotypes. They seriously want you to look at a character like Ivy and feel empowered.
The closest that any game company ever got to making a female character that doesn't fit a stereotype was Samus. Which they quickly fucked up in Other M by going "wait, she doesn't need a MAN telling her what to do? Let's fix that, and make her mentally weak, too."
In conclusion: Yes, the women are stereotypes, but that's the point of Duke Nukem. Duke's not about making a fair and honest depiction of everyone, and if it were, it wouldn't be Duke. Can we -please- focus on fixing all of the other games that make men into giant angry boxes and women into helpless stick figures with watermelons on their chests and asses?