A guy makes ONE lucky shot in the Death Star trenches, despite being mediocre-to-lousy at everything he does for the previous two hours, and suddenly he's a Mary Sue.
Like you said, that training orb or whatever was sure kicking his ass pretty good for a while, there.
Luke was quite far from perfect, all the way through. Didn't do everything right. Hard to like sometimes. His whole arc is a struggle for balance; not just with The Force, but within himself.
We can't say things that aren't true about one character, just because we want to like a new character more. Ain't right, ain't fair.