Is it though? How is someone wearing a costume from a particular Asian country equating to stereotyping all Asians? Doesn't it come down to intent rather than just the act of wearing a costume?
The problem is Asians have a pretty unfortunate trend of being stereotyped and I can recall a few examples even in media that's meant to satirize it, like King of the Hill with a Laotian neighbor that Hank just assumes is either Chinese or Japanese
It's not even an accurate costume, we're not really sure ninja wore black anyway and were more often in disguise, not secret assassins like in a James Bond movie, I forget which one, but that was also a pretty bad Asian stereotype.
The act of wearing a costume at Halloween has a particular intent, which is not the same as if you were doing so with the acknowledgement or understanding that it's meant to mean something, while Halloween costumes are pure leisure, practically speaking. Even a cosplay would have more significance, if done with respect in some notion of understanding that it isn't accurate, but a Halloween costume is commercialized no less, probably the most damning aspect
I can agree if you're setting out to merely do something to belittle others - that's offensive, but it would be offensive even if culture or race weren't involved.
One can do this unintentionally, belittling is part of marginalization, particularly in systemic racism and internalized racism of black people thinking they have to adhere to stereotypes or they won't be accepted by the majority white culture, it's a whole problem I can't even begin to explain or even understand fully as a white person.
I think you will find that it wasn't just white people. *People* have been pillaging from any culture nearby. The Persians were doing it hundreds of years before the Romans, the Moors were in Europe at the same time as the Vikings, and the Khmers held most of SE Asia for a good portion of 500 years.
Colonialism is just associated with white people because of recent history. Well, excluding Japan of course, but lots of people seem to forget that.
The problem is that white people in their colonialist endeavors pillaged upon pretty much any culture imaginable, especially the British empire, which bled into American tendencies with manifest destiny, etc.
Just because it's more recent history doesn't mean that the white supremacist attitude and systemic racism that underpins it and influences even well meaning white people that don't hold ill will towards minorities but want to "help" them isn't a bad thing and should be thoroughly condemned, especially by people who want to move forward and not cling to outdated ideas about American exceptionalism or the idea that black people are perfectly fine in American culture when they're anything but.
Japanese nationalism was far more isolated, in no small part because they preferred to stick within that general area (and it wasn't exactly easy in early history versus when they started utilizing more modern means, but their nationalism and colonialism only overlap slightly, I'd argue)