It's already been answered repeatedly over the course of several works: Char is only good enough to win against skilled pilots that aren't of ace quality, but loses against aces all the time. Whenever he and Amuro were evenly matched or even when he had the advantage, he always lost.

When Amuro had the Gundam and Char had the Gelgoog, Char lost. The Gelgoog is considered statistically equal or superior to the Gundam in every category that matters.

When Amuro had the Gundam and Char had the Zeong, they stalemated but with Amuro having the upper hand constantly. The Zeong was a monster that was stronger than the Gundam, yet he still couldn't win.

When Amuro had the Nu Gundam and Char had the Sazabi, Amuro won. For all intents and purposes the two were roughly equal, yet Char still got his ass kicked even after Amuro had been worn down by Char's minions beforehand.