I ask because their leaders seem to reflect (albeit in sensationalized form suitable for fiction) tropes reminiscent of the "Noble Savage" stereotype in Euro-American Literature, which was accepted (when not created) by some learned Euro-Americans.
For example, the fictional King Mulu tamed wild animals and used them in battle, and the fictional King Wutugu equipped his soldiers with rattan armour which was effective protection.
Edited in order to add: If you want to describe how effective rattan armour really was in battle (and the role of Nanman ethnicities in spreading its use in China), then I would appreciate that also.
I can't answer about the history of rattan armour, it simply had no role in the three kingdoms or the Nanman of that time. I can hopefully provide a useful answer on the rest to what is a good question.
The novel portrayal of the Nanman had little to do with the Nanman or attitudes towards the Nanman (which could be a range of local groups) when it came to fighting or behaviour. They had no rattan armour, no special animals though they were famous horse-breeders, however you are right in a civilized gentleman is teaching the "noble savage" trope to it and it was certainly playing into a "ideal" of the good Chinese official educating the barbarian.
Of the local peoples, we know little from written records as China didn't record much about them, not even local histories like Chang Qu's, and modern archaeology has had to do a lot of the filling in: they did ancestor worship, spirit worship, horse breeding, traded with a lot of places like modern-day Vietnam and Tibet, diverse cultures with varying burial rituals, creating ceramics and other artworks, tattooing. Distance and terrain made them difficult for Han-China to assert full control over so paying off local magnates and leaders to keep them happy was the usual method (and pretending one had total control but don't ask too much).
Historically the Nanman were the one group of non-Han-Chinese Zhuge Liang faced and even then, he was mostly facing Han-Chinese magnates like Yong Kai and his successor Meng Huo. Yong Kai's revolt had been going on some time and had allied with Wu but Shu had been too distracted to launch a campaign, founding an Empire, fighting Wu after the loss of Jing, dealing with the aftermath of Liu Bei's death and establishing the new regime under young Liu Shan.
In 225 Zhuge Liang marched south, splitting his army into three with Zhuge Liang the main force, local man Li Hui going to Yuexi, Ma Zhong sent against Zhu Bao at Zhangke. As his subordinate won their battles, Zhuge Liang was helped by the assassination of the long-running rebel leader Yong Kai by Yuexi King Gao Dingyuan for reasons not known, Zhuge Liang then facing local magnate Meng Huo and winning repeatedly before Meng Huo submitted with Zhuge Liang mindful of Ma Su advice to win hearts and minds.
Zhuge Liang was able to, after the short campaign, use a mixture of force, senior officials handling the area, bribery and splitting local authorities to prevent anyone being too strong, to keep access to the horses (while denying them to Wu who had been getting them via Shi Xie of Jiao), taxes, soldiers and mineral wealth of the region. There were still conflicts but nothing Shu forces under figures like Ma Zhong couldn't handle.
As Zhuge Liang's first independent command, providing (relative) stability to the borders and raw materials including mines, horses and manpower that funded Zhuge Liang's campaigns against Wei, they were important but they were not the campaigns the novel covered.
Did any of the novel's treatment of the Nanman have a basis in history or even just beliefs? One or two names like Meng Huo are historical but switched to being of the Nanman. It is possible that the elephants and the hot climate may reflect Zhang Qian, returning from his journey through to the Western Regions, report Emperor Wu of the Former Han Dynasty from when Qian was Daxia/Bactria. He noted some items from China's west had got up there (translation, Bin Yang chapter 2)
When I was in Daxia, I saw bamboo canes from Qiong and cloth made in the province of Shu. When I asked the people how they had obtained such articles, they told me, "Our merchants go to buy them in the markets of Yandu." Yandu lays several thousand li southeast of Daxia. Their customs are much like those of Daxia. The region is said to be hot and damp. The inhabitants ride elephants when they go into battle. The kingdom is situated on a great river. We know that Daxia is located twelve thousand li southwest of China. Now if the kingdom of Yandu is situated several thousand li southeast of Daxia and obtains Shu goods, it seems that Yandu must not be very far away from Shu. Now an envoy to Daxia by way of the Qiang territory is in danger, as the Qiang people hate us; if we send them a little farther north, they will be captured by the Xiongnu. The road through Shu would be the most direct route, and without enemies.
Han envoys dispatched in 122 BCE never got as far as Yandu (in India) with locals along the route sometimes intervening to protect their own interests but the novel may well have borrowed from such reports. Bringing such rumours about a place onto a different people entirely.
Zhuge Liang doesn't need a first campaign win as had been changed into Liu Bei's military mastermind and had just outwitted an entirely fictional mass invasion from all sides. But it could serve multiple purposes including building up Zhuge Liang as the greatest mind of his time.
The chapters could go and wrote a campaign completely different from the rest of the novel with scary creatures, fantastical landscape, use of magic and strange equipment, it is a campaign against a very exotic foe and terrain that can't be found elsewhere in the novel.
Who could doubt Zhuge Liang's brilliance in overcoming such obstacles, on the very edges of China, that are so strange? Is he not comparable to the great Han general Ma Yuan (with the novel making sure that connection is made) and does not his success shine compared to the limited success for centuries afterwards to keep the region under control? People certainly seem to remember the chapters and people enjoy them in modern games (Dynasty Warriors have had them for a while and was quickly requested for Total War Three Kingdoms which did add them)