Cedar Mountain was botched - I can't and won't argue about that. The point was that he'd be on the move with vigor, not that he was incapable of making a bad decision.
However, two notes on his merits.
1) The Valley Campaign did a pretty good job at its intended goals, and Jackson was able to take just about all of his two divisions (his own and Ewell) to Richmond after it concluded - despite there still being Federals in the vicinity.
I'd say that counts as a credible performance. Perhaps not A+ material, but certainly comfortably better than average.
2) If Lee really and seriously had any lack of faith in Jackson's ability to command, sending him to capture Harper's Ferry was not a good idea.
You mind explaining how this all works out to Jackson the sort of okay?
On McClellan: In the contest of bumbling, Pendleton losing 4 pieces of artillery of the army's reserve was...probably the less inept.
McClellan should have been able to do more than that with the resources at his disposal.