As for ' America v. Japan ' when it comes to art, I'd argue America doesn't have a style
. It's simply to young a country, nothing originated here except for this.
Which, while rather cool, doesn't aspire to story telling.
They are both single examples of an art style. The Aztecs and other pre-Columbus civilizations had a well developed art style that remained used by the successive civilizations (in minor variations) one after the other and was used in story telling.
Different cultures create their own styles of art. The more cultures intermingle with each other, the more they influence each other. However, even then you can denote some trends that mostly are kept by the originating culture. If you read a lot comics from Spain, England, France, Greece, Australia, or other places with a heavy Americian influence on them, you will notice that the styles are different from the American comic styles. Often cultures develop more then one style.
Most often these differences are ignored these days thanks to globalization, even before WWII. They intermingled with each other and thus blended into what was generally considered the acceptable styles. Most of Asian had been taken over by Europe and the art in those places mostly changed to suite European tastes.
However, Japan remained mostly free of European influences until after the war, thus when they took the comic styles of the Americans, and added their own culture's style, it was brand new and different. It is different enough that can't just be considered another version of the accepted styles, yet close enough that it can't be dismissed as a different style altogether. That is part of the problem.
When it was introduced into America, it was only really accepted as cartoons and comics at first. Things the adults considered as silly things for children. The association has remained ever since. For them accepting manga as a serious art style would be the same as accepting a picture of Bugs Bunny as serious art.
Many of these art experts have never learned art styles that did not originate in Europe, and thus can't appreciate Asian styles of art. They were taught that some schools were proper art, and that all the rest were rubbish.
American-style comics are distinct from Manga, but not as good in some ways. People say that all the characters in Manga look the same. When I look at most American comics these days, I see a few basic body types used over and over. They say Manga styles of dress are ridiculous. I say how are they any more ridiculous then a woman wearing high heals and a costume that covers less of her body then some bikinis, going into a fight?
Here is an idea, if a teacher ever tells you to "draw American" ask for a demonstration as you are not sure what the American style of draw is. Provide examples, like DC, Bugs Bunny, Scooby Doo and more.