The main difficulty of understanding the perception of Barbarian had of Romans is that every contemporary source was wrote by Romans. There is no real Iberian, Gaulish, British, Irish, Germanic, Sarmatic, Hunnic or Maur author having written down how they considered Romans, how much individuals Romans as a culture were differentiated from their state, etc.
Holding a Roman coin, an Aedun noble, a Germanic mercenary, an Iberian trader, a British freeman, etc. would have most likely felt differently, maybe radically so, even living in the same century : these feelings are still essentially unknown to us.
Roman (Latin or Greek) authors are our only contemporary literary source on this : it is not that the Barbarian views on Romans was necessarily less flawed that the Roman perception on what Barbarian thought of them, but this represent a lack of cultural depth and a certain selective bias especially when sources comes from campaigns accounts such as the Agricola or the De Bello Gallico. Even more "geographical" works such as Tacitus' Germania, are rather about informing Romans and confronting their preconceptions, and not "giving the floor" to Barbarians.

On the first part can be found general statements from the author about the resentment of populations during the conquest or thereafter.

The leading men of Gaul, having convened councils among themselves in the woods, and retired places, complain of the death of Acco [A chief of the Senones, brutally executed by Caesar in a roman fashion]: they point out that this fate may fall in turn on themselves: they bewail the unhappy fate of Gaul; and by every sort of promises and rewards, they earnestly solicit some to begin the war, and assert the freedom of Gaul at the hazard of their lives. (BDG VII)

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All the chief men of the Iceni, as if Rome had received the whole country as a gift, were striped of their ancestral possessions, and the king's relatives were made slaves. Roused by these insults and the dread of worse, reduced as they now were into the condition of a province, they flew to arms and stirred to revolt the Trinobantes and others who, not yet cowed by slavery, had agreed in secret conspiracy to reclaim their freedom. It was against the veterans that their hatred was most intense. For these new settlers in the colony of Camulodunum drove people out of their houses, ejected them from their farms, called them captives and slaves, and the lawlessness of the veterans was encouraged by the soldiers, who lived a similar life and hoped for similar licence. A temple also erected to the Divine Claudius was ever before their eyes, a citadel, as it seemed, of perpetual tyranny. (Annals, XIV)

An interesting and equally present variation is the warchief's passionate speech on why they're fighting Romans before the battle.

Critognatos, Arvern noble, during the Siege of Alesia

I shall pay no attention to the opinion of those who call a most disgraceful surrender by the name of a capitulation; nor do I think that they ought to be considered as citizens, or summoned to the council. My business is with those who approve of a sally [...] What, therefore, is my design? To do as our ancestors did in the war against the Cimbri and Teutones, which was by no means equally momentous who, when driven into their towns, and oppressed by similar privations, supported life by the corpses of those who appeared useless for war on account of their age, and did not surrender to the enemy: and even if we had not a precedent for such cruel conduct, still I should consider it most glorious that one should be established, and delivered to posterity. For in what was that war like this? The Cimbri, after laying Gaul waste, and inflicting great calamities, at length departed from our country, and sought other lands; they left us our rights, laws, lands, and liberty. But what other motive or wish have the Romans, than, induced by envy, to settle in the lands and states of those whom they have learned by fame to be noble and powerful in war, and impose on them perpetual slavery? For they never have carried on wars on any other terms. But if you know not these things which are going on in distant countries, look to the neighbouring Gaul, which being reduced to the form of a province, stripped of its rights and laws, and subjected to Roman despotism, is oppressed by perpetual slavery. (DBG VII)

Calgacos*,* before the Battle of Mons Grampius (Agricola)

Whenever I consider the origin of this war and the necessities of our position, I have a sure confidence that this day, and this union of yours, will be the beginning of freedom to the whole of Britain. [...] But there are no tribes beyond us, nothing indeed but waves and rocks, and the yet more terrible Romans, from whose oppression escape is vainly sought by obedience and submission. Robbers of the world, having by their universal plunder exhausted the land, they rifle the deep. If the enemy be rich, they are rapacious; if he be poor, they lust for dominion; neither the east nor the west has been able to satisfy them. Alone among men they covet with equal eagerness poverty and riches. To robbery, slaughter, plunder, they give the lying name of empire; they make a solitude and call it peace

Boudicea, (Annals)

[...]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. [...] 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

Cassius Dio (92)

You have learned by actual experience how different freedom is from slavery. Hence, although some among you may previously, through ignorance of which was better, have been deceived by the alluring promises of the Romans, yet now that you have tried both, you have earned how great a mistake you made in preferring an imported despotism to your ancestral mode of life, and you have come to realize how much better is poverty with no master than wealth with slavery. For what treatment is there of the most shameful or grievous sort that we have not suffered ever since these men made their appearance in Britain?

You might notice that the same ideas come a lot : preserving ancient freedom against tyrannic foreigners imposing their ways, refusal of slavery and the loss of individual and "natural" liberties and not just communal ones, Roman rapacity and expropriation are systematically present. How much these accounts, regardless of the sincerity, its lack thereof or anything in-between of the author, didn't fit a Roman trope of the Barbarian as a mirror held to themselves? Appian, for instance, while writing the Celtiberian Wars, regularly points how Roman behaviour was shameful but never posit the conquest was illegitimate : the point there might be that Romans ought to be more gracious conquerors, not driven by rapacity but by a certain mission civilisatrice everyone would benefit from morally.

Yet, such accusation shouldn't be dismissed as mere inventions or poorly-hidden agenda, but rather as defusing the reality of Barbarian accusation on a moral level, rather than systematical. For instance, Cicero defending Fonteius for his corrupt and brutal administration of Roman Gaul, less by evidencing his good government than asking Romans not to give much credit to former defeated peoples, necessarily lying and defaming their vanquishers. Grief, and vocal ones, were much less appreciated in political and administrative grounds than narrative, but both might have a same origin : peoples seeing Romans as a rapacious people that threatened their freedoms both as peoples and as individuals.