Mac has a below average arm but has excellent accuracy and timing in his throws. He has surprisingly good mobility and is very intelligent.
His fatal flaw has been a high propensity to panic under pressure. He doesn't succeed off platform not because he is unathletic but because he freezes up when forced to go off script. That's what every opponent figured out. Pressure and confuse Mac and he will become a dumb QB with a weak arm.
I don't know if his panic problem is fixable even with superior coaching and cumulative experience. But if he does master himself, he'll be a lot better QB than many folks here think.