"Angry White Boy" is a derogatory term for late '90s / early '00s nü metal generally referring to how the only people who liked it were privileged white kids who didn't really have anything to be genuinely mad about. The title isn't so much a reference to the musicians as much as the fad, which, sadly included some acts that didn't quite fit the description.
It's worth pointing out that Al did this polka around the same time that he worked with Ben Folds on "Rockin' the Suburbs," which was a direct attack at Korn.
Paul McCartney disapproved of Weird Al's "Live and Let Die" spoof "Chicken Pot Pie" because... Paul is a vegetarian? What sense does that make? Weird Al is a vegetarian too!
For one, Al wasn't yet a vegetarian when he first wrote the song. Also, Paul (well-known for his social crusades) is one of those vegetarians. In fact, I'm pretty sure he went full-on vegan after Linda died.
Why does "Dare to Be Stupid" have the line: "You can eat a bunch of sushi and forget to leave a tip?" That would be rude, to be certain, but not really stupid.
Because next time you go in there, they might just remember you as a bad tipper, and not be quite so careful to give you fresh sushi, I'm guessing.
It could also be that the stupid part isn't the lack of a tip itself but instead the "forgetting" part.
Actually, in Japanese culture the rude thing to do is to leave a tip, for some reason. So it actually becomes smart once you know this. It seems to me that Al was just writing the lyrics off the top of his head for the sake of funny.
Why, exactly, did Weird Al never do a parody of a 3 Doors Down song? Sure they're (relatively) obscure now, but from around 1999-2005ish you almost had to go out of your way to avoid them.
Because the thought never crossed his mind?
Because he couldn't come up with a good way to either parody or adapt one of their songs?