I don't think they're mindless savages, but they were very powerful men who were headstrong, proud, used to head-on fighting and quick and brutal combat. When you're in the heat of battle, the adrenaline is pumping, you're angry, you want to kill someone - there isn't much cunning thinking, or examined strategy while sipping on a cup of tea determining how best to defeat your opponent. You see a little man hiding behind a big shield, you think 'Ooo I want to hit that as hard as I can to make it break', underestimating the deadliness of the Gladius despite its short reach. The Celts were great strong men, excellent individual fighters, and all of that. I'm sure their IQ's were comparable to the average Roman. Though, I won't let you sit here and dream of big, muscly men with their dinguses hanging out and tell everyone that even unarmored they'd whoop-ass on a Roman Legionnaire because they wouldn't. If that were the case three times the number of these brave, cunning, amazing fighter Gauls should be able to triumph over a Roman Legion a third their size in numbers. Considering how you seem to think that each Roman legionnaire needs to beat three warriors who are better than him in combat to annihilate the other army. Or does pure organization make up for that much?
Face it, the Celts were unorganized, undisciplined, and suffered from poor strategies and poor leadership. No matter how brainy you think these guys are they would try and storm Roman pallisades in the thousands, hoping to carry the forts away out of pure brute force. The same way an individual Celtic warrior might try to overwhelm a Roman Legionnaire's shield through brute force.
It wasn't that they were dumb, they just didn't understand the more refined aspects of making war, conducting battle, and fighting at large because they'd never really had to fight wars or conduct battles like they did when they fought Caesar. That isn't saying they're dumb. It was like asking someone to do calculus before they'd attended a pre-cal class.