You have not tasted servitude. There is no land beyond us and even the sea is no safe refuge when we are threatened by the Roman fleet....We are the last people on earth, and the last to be free: our very remoteness in a land known only to rumour has protected us up till this day. Today the furthest bounds of Britain lie open—and everything unknown is given an inflated worth. But now there is no people beyond us, nothing but tides and rocks and, more deadly than these, the Romans. It is no use trying to escape their arrogance by submission or good behaviour. They have pillaged the world: when the land has nothing left for men who ravage everything, they scour the sea. If an enemy is rich, they are greedy, if he is poor, they crave glory. Neither East nor West can sate their appetite. They are the only people on earth to covet wealth and poverty with equal craving. They plunder, they butcher, they ravish, and call it by the lying name of 'empire'. They make a desolation and call it 'peace'
I've seen other translations that phrase it a bit differently but this the quickest I could find via Google
There's nothing unusual with historians, and especially Roman historians, being critical of their society, despite the 20 year rule here :p. Sallust, our earliest historian surviving in anything like a complete work, already thought Rome sucked and was expressly writing about why it sucked. Livy says he's writing because things for him suck so bad he doesn't even want to think about it (he's lying - counting by book he wrote a ton more about recent history than the distant past, but still, that's what he says).
Critiques of Roman imperialism aren't unusual either. For Sallust, the reason Rome sucked was (depending on which part of Sallust you read), Rome beat Carthage and without a dangerous enemy, Roman morality had nothing to keep it in place, or Rome conquered the East, and all that money coming to Rome suddenly just ruined the nobility. Depending on whether you think Virgil is pro- or anti-Augustus, you might see the description of Rome's empire sine fine and the injunction to Aeneas to war down the proud as a tongue-in-cheek indictment. And also there is Augustus' supposed command to not expand the limits of empire past the Rhine.
So there is a find tradition of critique of imperialism Tacitus had to draw on, but this, you must remember, is direct speech Tacitus set in the mouth of a barbarian. The convention of the ancient historian, even when the speech is available (and we know Tacitus had access to some speeches he rewrote anyway) is to write in a nice rhetorical set piece what a character must have said in the given circumstance.
This does not have to be a real critique of Roman imperialism Tacitus wished his reader to reflect on; all it has to be is what Tacitus thought a leader on the losing end of Roman imperialism might say about it when building an opposing coalition.