Well you can blame
Tacitus for that, he pretty much created the "Romans evil, Germanic tribes good" narrative in his book, in 99AD. "Germania" (Land of the Germani) is the book partially responsible for the idea of the superiority of the Nordic/Germanic "races" because it describes the Germanic peoples as actual "better" people who just happen to have inferior technology.
This kind of baffles me, cause reading it, I don't feel like Tacitus is giving the Germans that much praise. It's not so much a "Romans evil, Germans good", he oftentimes lambasts them for being primitive drunkards and thugs. He does praise their strength (especially in battle) and some of their virtues, but here, too, he points out their weaknesses at times.
For instance, in Chapter 4, he states that the Germans are great for a wild attack, but their strength is not suitable for prolonged work. In general, he seems to describe the Germans and their lives pretty fairly, but there are some comments here and there that seem less benevolent. In chapter 14 and 15, again, he calls out that the Germans are lazy and that they'd rather go to war to murder someone and steal whatever they need then to labor to make it themselves.
Frankly, it appears to me that some of the supposedly positive things within the book are actually neutral or even critical of the Germans, but to a modern reader it might seem like benevolent "noble and pure savage" stuff, caus a modern reader lacks the background of cultural attitude towards "barbarians" and what made a barbarian a barbarian from a Roman's point of view.
In Chapter 5, Tacitus writes that the Germans barely use silver or gold and if you find a container made of such metal, it's mainly a gift from a foreigner and it's used like it was made of clay. That is pretty clearly Tacitus taking the piss out of the primitive Germans for not even knowing the true value of such metals, it's "pearls before the pigs" so to speak. He further elaborates that only those that have been civilized by trade with Romans, who know that precious metals are... well... precious. And still, he continues, they prefer silver coins over gold coins, since it's more suitable to their humble needs.
In a modern perception, them not fussing over gold might be a sign of purity and pragmatism, to a Roman reader of the time, it might have been a rather clear cut criticism of their barbarism.
Another more veiled criticism might be Tacitus pointing out that the German tribes don't have cities, instead they live close to forests, wells or rivers (he doesn't say this, but compare it to the wonders of aqueducts in Roman cities or their elaborate infrastructure in general, even with smaller settlements). He openly criticises how Germans build their small settlements (directly comparing them to Roman ones of similar size) and points out that everything is made of wood, nothing made of stone and he calls them inelegant and artless.
There's stuff that gets praised, for sure, but there's oftentimes caveats with that, too. The German's prowess with weapons for instance is tampered when you consider that Tacitus explains that they have barely any armored fighters, their spears use thin blades of small size at their tips, that the Germans fight in family-groups and not as an organized army, that they are easily blinded by their rage (and how that can be taken advantage of). It's basically an elaborate description of Germans being great warriors but shitty soldiers that can't stand their ground against a well-disciplined, well-equipped Roman army.
One thing he really praises is that women are untainted by theatre plays (then again, that might be another jab at the primitive Barbarians, who don't even know theatres?), raunchy feasts or the exchange of secret letters (again: Maybe "lol Germans can't read"?). The one thing he seems to praise is how big of a deal faithfulness for women is. So yeah, there are words of praise that take the piss out of Rome by extension (or some other neighbors to Rome, for that matter), but overall, that's just a small part of his book, I'd say.
tl;dr: To say Tacitus' book is a loveletter to the German tribes is an overstatement. He doesn't uniformly praise them and even some of his praise seems like a thinly veiled jab. There is some stuff where he outright calls them uncultured savages and a lot of instances where such an attitude might be hidden behind nice words.
Overall, it's a Roman writing about Barbarians, so while he might have praise for some aspects of their lives, the superior position of Rome above all is what this text is most likely trying to underline, even despite the few instances of him praising Germans, they are vastly outnumbered.
