You imply that his dialect adjustment is a smoking gun indicating false character. While this charge may on some level be aimed at the entire acting profession -- and its audiences -- it doesn't make sense as a means of "piling on" to an actor when he's down, since his profession demands a great ear.
Some people are just natural "language people", experiencing language the way a musician experiences music -- with the ability to hear the perfect pitch and the subtleties that each language contains uniquely. Other people maintain their original accent for life, even if it is broken English and they have lived in this country for decades.
The Australian actor Simon Baker speaks the two dialects in public -- American on his hit show "The Mentalist" and Australian in chat show appearances, but is gradually losing his Aussie accent the longer he stays here.
I personally modified my southern accent when I went to college at age 17 in a northern state and was teased over it -- something Gibson undoubtedly encountered if he returned to America at age 12. I adopted what I call "television announcer" as my dialect, and stayed with it as long as I was working professionally. When I lived in an Italian-American neighborhood, it was also useful to learn the Sicilian-inspired dialects you hear on The Real Housewives of New Jersey for dealing with local officials, proprietors and neighbors. Now having retired to Maryland, I'm reverting to Marylandese for casual conversation. I'm also workmanly fluent in several other European languages and use them in my retirement job dealing with the public. "Interesting"? Or just a practical adaptation?
I thought Mel Gisbon’s Soliloqy of Shylock from the Merchant of Venice in his portrayal of a deformed man in the film -— THE MAN WITHOUT A FACE was quite impressive.
Watch this 30 second clip (Starting from 0:37) :
I wasn’t implying any lack of character by the fact Gibson lost his Australian accent. He probably wanted to sound more American which is understandable since he was born one.
By the way, Gibson went to AU when he was 12, returning to the US as an adult.
I have a friend who travels alot for his job. He said that in Beijing at Tianamen (sp) Square, young Chinese students look for true Americans just to walk around with so they can practice their English. It seems that American "television announcer" dialect is able to land them good jobs after they graduate.