I never consider any of the post-apocalyptic movies Dieselpunk. I consider them post-apocalyptic... There was no style except for grunge in the movie. I've never really bought into the classifications of Dieselpunk. Star Wars is more Dieselpunk than Waterworld...
Just because it's post-apocalyptic and has some diesel powered vehicles makes something Dieselpunk? There is absolutely nothing in either Waterworld or Mad Max that's in the least Dieselpunk. As the Home page to the Forum states the genre of Dieselpunk, "blends the aesthetics / pop culture of the 1920s - 1950s with today." Neither of those movies have any of the aesthetics of the Diesel Era.
There's very little Post-Apocalyptic Piecraftian Dieselpunk to choose from. One could say that the animated movie Wizards with its heavy use of Nazi imagery with the mutants and Blackwolf's dream machine is Dieselpunk. One could also say Blade Runner is Post Apocalyptic Dieselpunk because the Earth is dying and all's that's left on Earth are those unfit for travel to the Off-World Colonies yet it also has a heavy dose of Diesel Era aesthetics.
Here is my comparison.
Waterworld - 20-50 US
Lack of supplies, essentials, and in general a greatly enhanced difficulty in life i.e. Dustbowl
Lack of supplies, essentials, and in general a greatly enhanced difficulty in life i.e. Waterworld
The alienation and condemnation of those not belonging to an established people. Both present in 20-50 US and Waterworld.
Neutrality and Conflict
A lone power that wishes to remove itself from conflicts involving outside powers. Again present in the aforementioned US era and Waterworld. Neutral power pulled into conflict due to aiding another power, ditto. Conflict ending with the massive use of destruction of the "enemy" power, present in both again.
A previously "defeated" power is again brought forth to fight those that do not agree with their choices. The Third Rich in WW2 and Deacon makes references to the Exxon Valdez spill (okay this was a Hollywood attack against pollution but it is still comparison.)
I've been told that I sound like a dark Ottensian though I think I like Piecraftian more (I'm a former goth and a 80's child forgive me) So to me there is a very post apocalyptic flavor to Dieselpunk. But that is my view on it I am just as right or as wrong as the next guy. Personally I don't think the fashion has an impact on my choice of if a movie is Dieselpunk or not, though the look of the movie may (Casshern's fashion is fare more cyberpunk but the story and look ,to me, is total Dieselpunk). Waterworld does not look dieselpunk it is the story I think places it as such. Now as to Mad Max that would be left to a different discussion entirely this is for Waterworld, until I find the mad max discussion, I'm slowly making my way through the site.