With her exotic features, bronze skin and biracial heritage, the newly crowned Miss Universe Japan has stirred up a storm of controversy for not being "Japanese enough".

Born to a Japanese mother and African-American father, 20-year-old Ms Ariana Miyamoto may speak and behave like any other Japanese girl her age, but critics think she "has too much black blood in her to be Japanese", as one online comment puts it.

Despite the popularity of Western beauty standards (think dyed blonde hair and fair skin) and pan-Asian faces dominating fashion magazines and TV, Japan's largely homogenous society still clings on to traditional ideals for beauty queens representing the country.

And hafu, or half Japanese people, are simply not good enough by their standards.

Ms Miyamoto had ignited hopes of Japan embracing racial diversity when she became the first mixed-race Japanese to win a local pageant earlier this year.

But when she was crowned Miss Universe Japan, netizens took to Twitter to question her win - whether out of anger, surprise or sheer bewilderment.

Some said her face looks "too gaijin (foreigner)", while others felt it was a "contradiction" to pick a hafu over a "junnihon (pure Japanese)".

According to Kotaku.com, one netizen even wrote: "It makes me uncomfortable to say she's representing Japan."

Japanese people like to think their unique culture is "inaccessible to foreigners", according to Harvard University professor Theodore Bestor, who specialises in anthropology and Japanese studies.

"One of the ways in which Japanese think of their own society as 'unique' is to emphasise the homogeneity of Japanese society," he was cited as saying by the Washington Post.

Ms Miyamoto is not the first Japanese beauty queen to draw flak for being "unJapanese".

Ms Riyo Mori, the first-ever Japanese to win the Miss Universe title in 2007, was also criticised for being not kawaii (cute) enough and being "arrogant and dumb", according to reports.

After her win, Ms Miyamoto had to "explain herself" to the Japanese media, emphasising that she is indeed a Japanese citizen, born and bred in the country.

But growing up with exotic looks was not easy for the 1.73m-tall beauty.

"In school, people used to throw garbage at me," she said in an interview with CNN.

While there are haters out there, there are also supporters who sent Ms Miyamoto encouraging messages like, "Don't lose to discrimination" and "Having a different ethnicity in you doesn't make you any less Japanese", as cited by RocketNews24.

Biracial filmmaker Megumi Nishikura has also defended the new Miss Universe Japan, telling NBC News that her win "is a huge step forward in expanding the definition of what it means to be Japanese".

"The controversy that has erupted over her selection is a great opportunity for us Japanese to examine how far we have come from our self-perpetuated myth of homogeneity while at the same time it shows us how much further we have to go," said Ms Nishikura, who co-directed the 2013 documentary Hafu: The Mixed-Race Experience in Japan.

About 20,000 mixed-race babies are born in Japan every year, and one out of 30 babies are born to parents of whom one is non-Japanese, according to official data.

Undeterred by the wave of criticism, Ms Miyamoto is determined to train harder to compete in the international Miss Universe pageant next year.

"I want to tell the world, even a half-Japanese can represent Japan," she told CNN.