I'm assuming that we're talking about Tacitus the politician and historian, and not Tacitus the emperor, who claimed to be descended from the former.
Of those works which survive, Tacitus appears to have a highly critical and skeptical view of the empire, imperialism, and just about everybody in it. What may be his most famous quotation is "where they make a wasteland, they call it peace." This is from an imagined dialogue between a Roman and Briton that may be a good source for insight into the nuances of Tacitus' opinions (linked below).
Tacitus was not so critical of empire or imperialism as he was of tyranny and abandonment of one's duties. The context of Tacitus' life probably made him aware of the privileges he was born with, and his close relationship to Agricola probably influenced his views on military imperialism in favor of Rome. Additionally it would have been virtually impossible for him to generate a political career if he just went around criticizing everything and never compromising. Thus Tacitus, by birth and choice, was very much an endorser of the Roman way of life. For example, in the speech linked below, Tacitus offers criticisms of Rome through the "barbarian" speech, but it is clear that he personally feels the Romans are the better of the two.
Tacitus often focuses on the contrast between men and institutions or ideas, to put it broadly. He rarely condemns or elevates any individual but is equally critical and complimentary when possible. He seems to view the empire of his period as a virtuous deliverance of culture and civilization to non-Romans, but within the empire, the material benefits and political nuances that grew fundamentally from being an empire were having detrimental effects. Tacitus felt that the senators were abandoning their duties as critics and speakers in order to curry favor with the emperors, while the the power of the emperors was fleeting, their favor unpredictable, and their existence dependent upon the army. Meanwhile, the general populace was beginning to forget its obligations to moral living and instead sate itself with luxury; he even describes the populace reacting to an invasion of Rome as if it were a new form of entertainment.
It should also be considered that Tacitus probably knew that his own life depended upon carefully negotiating his political relationships, and that he may have been less critical in his writing than he actually felt. His writing overall is known for being relatively dispassionate, and he may have used this as a defense against accusations of personal bias.
It would appear that Tacitus really had little issue with imperialism; he did have a problem with the way imperialism was being played out in Rome, and how power was regularly abused by the emperors of his lifetime. He saw its state as a degenerate one, corrupted by its own successes, and yet this was still, in some ways, better than the chaos rampant in the world outside Roman rule.