Actually, I haven't seen a single historian voice that opinion. It's not as if the battle of Gettysburg was the only achievement in his life. He rose to become a brigadier general and then a major general, forsaking a private career for one of public service. He later went on to become governor of Maine and president of the Bowdoin College. Eventually, Congress gave him the medal of honor for gallantry in his service at Gettysburg. His courage is undisputed as he was wounded several times on the battlefield, often thought to be mortal wounds, only to come back to fight again.
Ironically, Chamberlain may be best known for his graciousness in victory. A few weeks after Lee's surrender at Appomattox, Chamberlain, and his troops were poised to receive the surrendering confederates. As major general John B Gordon, known as the fiery commander of the confederate second corps rode by with his head bow and eyes cast down Chamberlain was moved by the momentous nature of the occasion. He ordered his bugler to have his troops "order arms" and "carry" (that is how marching soldiers would salute a dignitary at the time) as the confederates passed. General Gordon recognize the bugle call, his head snapped up, and he turned around and ordered his troops to reciprocate the gesture. What had begun as a humiliating ceremony of surrender morphed into a statement of mutual respect. Probably not the act of a "self-aggrandizing blow hard", but entirely consistent with a medal of honor winning "officer and a gentleman."