What aspects of Germanic tribal life does Tacitus seem to admire and/or criticize? In the process of doing so, what might Tacitus seem to imply about the "barbarian" within Romans of his own time? Were there ways that even the Romans might learn from the barbarians?

Tacitus (56/57–125) was a Roman historian. We do not know much about his life, but we do know that he served in the Roman government for years before turning to writing. One of his works is Germania (98).
One thing he admired about the Germans was their pure race and distinct character. They had not intermarried, so there was a "family likeness" among them.
Tacitus also thought highly of their military prowess. The German infantry, in particular, was magnificent. They were agile and honorable warriors. It was their duty, Tacitus wrote, to never lose their shield in combat.
Another commendable characteristic of the Germans was their hospitality. No house guest was refused entry. Strangers and acquaintances were treated equally well.
The thing he admired most about the Germans was their marital fidelity. One wife was the norm, and adultery was very rare. Tacitus admired this because of his dislike of the debauchery that was all too prevalent in Rome.
Not all of Tacitus's observations were positive, however. First, he noted that the Germans were very superstitious, and that they were "addicted to divination by omens and lots." Second, the men were indolent when at peace. They slept, ate, and drank too much, and "stupef[ied] themselves in inaction."
On the whole, Tacitus admired the Germans and thought the Romans could learn some things from them.
https://www.gutenberg.org/files/7524/7524-h/7524-h.htm

In Germania, the Roman senator and historian Tacitus provides a detailed examination of the German tribes around the year 98 CE. He never visited these tribes himself and relied on the accounts of others. His writings describe both the land and customs of the German peoples. It is certainly colored with interpretation. Tacitus does not hesitate to draw comparisons between the German tribes and his fellow Romans.
For instance, Tacitus is quick to praise the marriage customs of the Germans. He holds a favorable view of their practice of monogamy and fidelity. He seems to insinuate that the Romans can learn a lot from this.

Their marriage code, however, is strict, and no feature of their morality deserves higher praise. They are almost unique among barbarians in being content with one wife apiece—all of them, that is, except a very few who take more than one wife not to satisfy their desires but because their exalted rank brings them many pressing offers of matrimonial alliances.

Tacitus also is eager to praise the childrearing methods of the Germans. He notes that mothers breastfeed their babies themselves instead of having nursemaids attend to that task. He praises the Germans for waiting to full adulthood to marry and have children. He credits the robustness and health of the German children to this practice. Reading between the lines here, we can suppose that Tacitus is criticizing the licentious ways and apparent feebleness of aristocratic Roman children by praising the Germans for these practices.
Tacitus does criticise the Germans for their fondness of beer and frequent drunkenness. He also calls them lazy and barbaric on several occasions.
Despite this, Tacitus consistently praises the Germanic tribes for their simple yet pure way of living. By the end of the first century, many conservative Romans were criticizing their people for what they considered immorality and a loss of traditional Roman virtues. Many of the values that Tacitus ascribes to the Germans are comparable to the values of the first Romans as described by other historians, such as Livy, and poets, such as Virgil. It is possible that by describing the Rome's old barbaric enemy as virtuous, Tacitus is delievering an underhanded rebuke of the Romans of his day.