Maybe it could have been taken but it sure wouldn't have been easy. Union Artillery commander Henry Hunt had been on Cemetery hill all afternoon placing his guns and would have rained hell on any attacking forces. We've seen too often what happened to soldiers in the civil war attacking an uphill position against artillery. Further, Ewell's army was pretty played out by that time, which was the excuse Ewell and Early gave for not attacking.
But Jackson would have attacked that hill. Jackson, like Grant, realized that if you give the enemy time to fortify on good ground the results are much worse than going right then with what you've got, despite an apparent mismatch. It was only going to be worse the next day so if they wanted to "take that hill", this was the time to do it. Damn the torpedoes. But that's what makes Gettysburg such a fascinating battle. So many "what ifs".
A good “alternate history” of the Battle of Gettysburg was written a number of years ago by military historian Peter G. Tsouras. Its title is “Gettysburg: An Alternate History.” I highly recommend it. In it he has Culp’s Hill being taken, among other changes, all of which have been discussed via “what if......”