Well...I guess it would be pretty messed up if I didn't wake from the dead to post in this thread. There's probably less reason to be upset about Jet's casting than any other character. Jet's voice actor in the English dub of the original show is black, so at least there's some measure of consistency there.
The argument over whitewashing Spike was and is dumb for several reasons. Spike never had to be of Asian descent. It wasn't a must. Now...there are plenty of hints as to Spike's background, but to my recollection, the show never explicitly states what race/ethnicity any character is (though plenty of hints for Faye, too). I always looked at it as the anime saying that the movement of humanity to space rendered a lot of the former racial boundaries moot, and the ethnic background of each character therefore did not really define them any longer - all of the characters were wanderers without homes. The most they come right out and say of Spike is that he was born on Mars. That's it.
For instance: let's assume Spike is a white man from a Jewish family. He was born on Mars, worked for a crime syndicate that is seemingly Chinese, and was betrayed by his partner of totally ambiguous ancestry who wields a Japanese sword. It's a mishmash. The old stuff stopped mattering when Earth went kablooey, because all of a sudden everybody was in the same position of either being dead or needing to get the hell out. (By the way, there's an anecdote floating around the internet where, at a convention, somebody asked Bebop's director Shiniciro Watanabe whether Spike was Jewish. Supposedly, Watanabe replied by chuckling and saying "Sure, why not!" The creators of the show weren't obsessed with race, probably didn't give it much more than a few moments thought, and didn't build their entire story around it.)
That said...if the original source material was ambiguous, there was nothing really stopping Netflix from casting an Asian man as Spike, either. But about whitewashing one more time: how about the idea that Japanese people are only capable of or interested in telling stories about Japanese people (or other Asian folks...)? Seems...I dunno...kinda racist? That was Netflix's mistake - and that of every online journalist who had something to say about being happy that Spike wasn't whitewashed - if you ask me.
