A fundamental problem for getting an idea of the mindset of the "barbarian" people is their lack of writing. From archaeological sources it is obvious that there was trade between - , and probably some fascination of these people for -, the Mediterranean civilization. The first Roman (There were some Greek predecessors, but I can't talk qualified about them) to really write about the people outside the Roman empire was Caesar, and more significant Tacitus. One of the most interesting aspects of Tacitus' (living in the first and second century AD) Germania is that while it described the people as barbarians, his text also has an aspect of criticizing the Romans. The Romans, in his eyes, had become decadent and weak - while the Germanic tribes were maybe uncivilized, but honest, brave, and strong in the wilderness they lived in.

So Tacitus expressed a critique of the Romans through a description of the alternative. When you read him, you might be tempted to think: "If those Barbarians were like that they must have looked down at the Romans!" The problem is, for a long time this was exactly how Tacitus was read and his topos continued - but we don't really know what those Barbarians though themselves - as you pointed out, those people incorporated in the Roman empire quickly adapted and were romanized.

To make this problem a bit more clear I'll expand a little on other problems the lack of sources caused: many people and tribes that would turn up centuries later were identified with those that were already known from the texts. But it's e.g. hard to positively confirm, whether the Suebi in the fourth century were really descendants of those Suebi Tacitus talks of three hundred years before.

This was a problem for a very long time when coming in contact with an illiterate people. For example when the Magyars/Hungarians invaded the West-Frankish-Realm for the first time in the 10th century the sources, such as Widukind of Corvey's "Saxon Chronicle" refer to them as Avars or even as "Those Avars who are called Hungarians" - which was a complete different people that had been defeated by Charlemagne. But the chroniclers of the time of course used the terminology they had available and thus mislabeled them.

So yes, the trope has a historical basis, but it's doubtful that it stemmed from the "Barbarian" people themselves.

The historian Tacitus is likely responsible for this trope, if not wholly, then at least in part, largely due to the popularity of his works and the political bias inherent in his writings. I refer primarily to his Annals, Histories, and, to a lesser extent, Agricola. Writing primarily during the Flavian dynasty, Tacitus takes a relatively harsh stance against the actions and excesses of the Julio-Claudians; he originated the idea that Nero "fiddled" as Rome burned. He actually recited a poem and played the lyre, others later interpreted it to fiddle. Regardless, Tacitus also uses his writings to complain of what he sees as shortfalls in the Roman aristocracy of his time, just in a more roundabout manner.

When writing of the Celts or other Germanic tribes, he describes simpler, more just, and more equal societies unburdened by the problems of Rome. It is in his works that the idea of the "noble savage" first has its genesis. One of his more interesting depictions is of the defiance and capture of Caratacus, the Briton leader that rallied resistance against the 43-44 AD Roman invasion and led the guerrilla effort against Rome until 51, when he was captured not as a result of Roman competence, but betrayal. After claiming Claudius exhalted the defeated Briton almost as highly as himself and describing Caratacus as well-known and admired throughout the empire, credits him this speech:

"When he was set before the emperor's tribunal, he spoke as follows: 'Had my moderation in prosperity been equal to my noble birth and fortune, I should have entered this city as your friend rather than as your captive; and you would not have disdained to receive, under a treaty of peace, a king descended from illustrious ancestors and ruling many nations. My present lot is as glorious to you as it is degrading to myself. I had men and horses, arms and wealth. What wonder if I parted with them reluctantly? If you Romans choose to lord it over the world, does it follow that the world is to accept slavery? Were I to have been at once delivered up as a prisoner, neither my fall nor your triumph would have become famous. My punishment would be followed by oblivion, whereas, if you save my life, I shall be an everlasting memorial of your clemency.' Upon this the emperor granted pardon to Caractacus, to his wife, and to his brothers." (Annals 12.36.1)

This is a subtle barb, but one none the less, and much of his work speaks as highly of the 'barbarians' in the same manner. Tacitus' description of the Boudicca rebellion ten years later places the blame for the uprising entirely on Roman atrocities against the Iceni, and once again exalts the Celt leader (Annals 14.29-37 has the account). He does speak highly, at times, of Roman discpline and bravery, but little else.

To continue on message, Tacitus even contributes speeches to Boudicca and the Legate Paulinus Suetonius before the final battle of the rebellion. Let's compare them and see which is more noble and uplifting:

"Boudicea, with her daughters before her in a chariot, went up to tribe after tribe, protesting that it was indeed usual for Britons to fight under the leadership of women. 'But now,' she said, 'it is not as a woman descended from noble ancestry, but as one of the people that I am avenging lost freedom, my scourged body, the outraged chastity of my daughters. Roman lust has gone so far that not our very persons, nor even age or virginity, are left unpolluted. But heaven is on the side of a righteous vengeance; a legion which dared to fight has perished; the rest are hiding themselves in their camp, or are thinking anxiously of flight. They will not sustain even the din and the shout of so many thousands, much less our charge and our blows. If you weigh well the strength of the armies, and the causes of the war, you will see that in this battle you must conquer or die. This is a woman's resolve; as for men, they may live and be slaves.

Nor was Suetonius silent at such a crisis. Though he confided in the valour of his men, he yet mingled encouragements and entreaties to disdain the clamours and empty threats of the barbarians. 'There,' he said, 'you see more women than warriors. Unwarlike, unarmed, they will give way the moment they have recognised that sword and that courage of their conquerors, which have so often routed them. Even among many legions, it is a few who really decide the battle, and it will enhance their glory that a small force should earn the renown of an entire army. Only close up the ranks, and having discharged your javelins, then with shields and swords continue the work of bloodshed and destruction, without a thought of plunder. When once the victory has been won, everything will be in your power.' (Annals 14.35-6).

The divide is obvious and the difference could not be more clear. There are many more passages of similar vein. Nor was Tacitus alone. As much damage as he did the Celts, Caesar implies a grudging respect for parts of their culture. Diodorius also writes relatively glowingly of them.

In final answer to your question, the ones responsible are the roman historians themselves, who used the actions against the native tribes to promote an agenda, or who may even have felt badly about the destruction of the culture. Toward this end, the Celts were presented as more simple, less concerned with affluence, and victims of conquest.