「社会通念」とそのまま訳するのであれば、"a socially accepted idea"とするのがわかりやすいですが、今回の文脈では"a common view"や"a generally received viewpoint" などと表現することも可能だと考えられます。
ちなみに、今回の質問の文章は以下のように訳しだせると思います。
例)It has [had] been a common view that Ariel should be played by a white woman.
The socially accepted idea was Ariel was supposed to be played by a Caucasian woman.