I don't view Oberyn as being an idiot there, though. He was absolutely enraged, and in that state your capacity to reason naturally diminishes, but it isn't a sign of stupidity. It's the way most human beings naturally respond to something tragic that has befallen them or their loved ones at the hands of another. Oberyn was fueled by vengeance and sought proper retribution, not only against The Mountain but Tywin as well, and that was the intent behind his words and actions once you go beneath the veil of his hatred.
Given what happened to his sister and her children, Oberyn's state of mind there is totally understandable. It's just unfortunate that his emotions got the better of him - in particular in a fight with someone like The Mountain - but I don't think it's a sign of idiocy or particularly out-of-character.
He was a likable character played by a strong, charismatic actor, which made his death rough (especially the gruesome fashion in which it happened), but I also think it was a terrific moment in the series from a purely dramatic standpoint. Oberyn's righteous rage combined with his highly effective, acrobatic fighting style, clearly superior in straight combat against a much larger, far more armored opponent others only speak of in hushed, terrified tones (classic David vs. Goliath). Unable to keep his emotions in check as he unleashes a torrent of hatred towards his bitter enemies, Oberyn doesn't defend himself properly against what was yet to be a fully defeated foe, and pays for it by being brutalized in a way viewers won't soon forget. It's harsh but excellent drama.
I hope not!
