A considerable body of readers considers Liu Shan to be Obfuscating Stupidity while being a Manipulative Bastard. Especially considering he had one of the longest reigns (41 years, in which 30 was after Zhuge Liang's death) in Chinese history in the weakest state in one of the most turbulent era, without being a tyrant, embroiled in a Succession Crisis (like Wu) or controlled by a Regent for Life (like Wei). His most Idiot Ball moment (claiming "I am too happy [in Luoyang] to think of Shu" after his surrender) was especially interpreted this way, as claiming to have any remorse towards Shu would most likely cost his head. As a matter of fact, he is also one of the few surrendered monarchs who died of a natural cause.
Historically, Liu Shan was never actually mentally retarded in the first place, nor was he dropped on the head by Liu Bei.
Some fans tend to look at Zhuge Liang in a less heroic light, seeing his nigh omniscient strategies and heavily pressuring Liu Bei into both taking over Shu and later declaring himself emperor as an attempt to rule China from the sidelines while someone else takes the glory/scorn in the process. Though a more specific case can be made for his treatment of Wei Yan. Whether or not he originally intended to rebel after Zhuge Liang's death or was driven to do so by Liang's plotting is left somewhat ambiguous. The badmouthing form of scrutiny he's received from the historian Chen Shou does not help his case.
Practically EVERY character can suffer this if you dare look at forums regarding their historical counterparts compared to their novel counterparts; some people received the bad ends of the scrutiny, while some are flat-out built from supposed folklore (or as some haters deem it to be, lies). In the original creator's eyes, it can be his own form of this trope as whole as well, due to how he expands the roles of certain characters and/or outright twists it apart from their actual recorded history.
The whole Shu Kingdom is not free from this, as explained with both Zhuge Liang and Liu Shan; the former has a ton of myths surrounding him, with historians such as Chen Shou outright bashing him for incompetence on certain fields if his word was to be truly believed. Liu Bei was big case as well in terms of being just and virtuous or putting up a front that was as treacherous as his historical self, as this division stuck with him even with his Dynasty Warriors-portrayal.
Badass Decay: In the first few chapters of the novel, Liu Bei displayed some serious fighting skills and was able to duel Lu Bu alongside his brothers. Later on, he became more of an Iron Woobie.
Broken Base: There's a camp who enjoys the story for what it is... and then there's another camp who's absolutely furious that Shu receives way too much Historical Hero Upgrade while in history they're not that virtuous or great, and other kingdoms who are more capable had their capabilities downplayed and thus often felt compelled to make statements about how the history is always the "more right version."
Crowning Moment of Funny: XI's tutorial. Specifically, its depiction of Liu Bei, and the fact that the characters breaks the fourth wall at times.
Designated Hero: Liu Bei is held up as The Hero of the novel; virtuous, benevolent, etc. yet he repeatedly abandons his wife and children, doesn't punish a hunter for murdering his wife to feed him, and also throws his own baby to the ground. The book doesn't call him out for them because of Values Dissonance and his effort to restore the "legitimate" dynasty.
It also does NOT help in that historically, Liu Bei was pretty treacherous during his time.
Ensemble Darkhorse: Ma Dai plays a small role in most of the book, but has the tendency to end up in every "Characters I want to see in the next Dynasty Warriors game" list. And voila, he did, for the seventh installment.
Ho Yay: Liu Bei is noted to sleep in the same bed as quite a lot of his trusted generals, brothers, or strategists. While this is a thing one does in ancient China to show respect, the novel also indicates that Liu Bei and Zhao Yun specifically showed great affection for one another, down to tearful farewells and, in one traditional story, singing poems to each other about "being joined as one body."
Cao Cao's the kind of guy who will recite poetry on the Yangtze, murder a nearby lackey who criticizes it, and then bury him with full military honors, all in the same scene. He's unfailingly polite to his guests, even when he's about to have them killed. He's unflappably cool, even after a defeat they'll be making movies about a couple thousand years later. The man is the original magnificent bastard.
Zhuge Liang, is known as a masterful strategist thousands of years after the time in which he lived. A character holding a fan of crane feathers makes them seem more magnificent, more brilliant etc. by association.
Sima Yi reaches this when he gives his speech to Gongsun Yuan's emissary:
"There are five possible operations for any army. If you can fight, fight; if you cannot fight, defend; if you cannot defend, flee; if you cannot flee, surrender; if you cannot surrender, die. These five courses are open to you, and a hostage would be useless. Now return and tell your master."
Zhou Yu whenever Zhuge Liang isn't also in the scene. Puts on an elaborate Feed the Mole plot, complete with a drunken sword dance. Hands Cao Cao his greatest military defeat at Chibi.
Mainstream Obscurity: Its importance in the Asian literary tradition is hard to overstate, and you can just scroll down below to see the extensive list of derivative works it's spawned, to say nothing of the numerous references to it, its events and its character that crop up so frequently in Asian culture, yet like Shakespeare and other comparable western classics, when it comes to people who've actually read it, particularly outside of a school context...
Nightmare Fuel: In the books, the nightmarish reign of Sun Hao. Like Caligula himself, Sun Hao was initially an impressive young ruler...but then the disgusting hedonism, torture and completely disproportionate retribution begins. Anyone who disapproved of his bizarre, prophecy-based politics would be executed, most likely in a horrendous way, including being boiled alive. When his brother was kidnapped to be used as a figure head for a rebellion, Sun Hao's forces defeated the rebellion...and then executed his brother, his mother and most of that side of the family just to make sure. At one point, one of his ministers sickened...and Sun Hao thought he was joking. So he had the guy whipped, sawed and burned alive. Said minister died under the torture. His whole clan was exiled. What a monster. Sun Hao can be considered the second coming of Dong Zhuo in the late Three Kingdoms without any of Dong Zhuo's small positive traits
One-Scene Wonder: Mi Heng. He only shows up in one chapter but his delivery of so much snark and of course his infamous "drumming naked" scene helps him make a lasting impression.
They Wasted a Perfectly Good Character: Xiahou Dun was historically known for being very insecure and irritable over the loss of his eye and despised his nickname "The Blind General" to the point where he would smash any mirrors in his presence. This is left out of Three Kingdoms (and almost every other major adaptation as a result) leaving him to have very few outstanding personality traits.
On the other hand, Xiahou Dun was not a particularly skilled general, and his record essentially goes from getting tricked to getting ambushed to getting tricked. Later in life, he was usually put away from the front and probably kept his job because of his relationship with Cao Cao.
Values Dissonance: This is not the 21st century. Events such as the value of one's body, and relationships (valuing of one's sworn brothers over wives and children) will only make sense in the light of historical Chinese culture.
He Jin could definitely qualify. After living through pretty much every plot by the eunuchs to make sure no royalty threatens their power, he receives an invitation to the Forbidden City. At the time, he is involved in a rebel plot and the eunuchs have eyes everywhere. In fact, pretty much every rebel warns him not to go. He brushes it off and decides to go anyway. So they insist on escorting him, well armed. Then, the guards tell him he must leave his guards at the door. He agrees, and proudly walks in. He, unsuprisingly, is surrounded by the eunuchs who hack him to pieces and toss his head back to his friends.
Lu Bu also qualifies. The man is, simply put, a naive fool. Seriously; he's swayed to work for Dong Zhuo in the first place, but that isn't the end of it. He's charmed by Diao Chan, and then later, goes back and forth between alliances because of what someone who wants to use him just happens to tell him. He slowly becomes smarter and craftier as time goes on, but that naivety still remains. His temper doesn't help either. The only thing Lu Bu had going for him was his incredible strength in battle, but even then, that just made more people want to use him, and that just led to a continuing string of Face Heel Revolving Door choices caused by his own naivety and gullible-nes. If it weren't for his sheer strength and skill as a warrior, he wouldn't really have gone anywhere in the Three Kingdoms period.
Zhang Fei certainly counts. He didn't put much thought into his actions, and just did whatever he wanted to regardless on the consequences. Case in point when he's supposed to be guarding the castle while his Oath Brothers are away. He's supposed to not take a single sip of wine, and also not beat his own men, and he even writes these very rules on the wall so that he won't forget. And what does he do? He drinks a ton of wine, and beatsLu Bu's cousin. What happens next is probably what you would expect: Lu Bu unleashes holy hell on Liu Bei's castle, and the only thing Zhang Fei can do is run away. He even tries to kill himself after this, though Liu Bei & Guan Yu stop him. So at least he's very sorry, but that still doesn't excuse what he did. Thankfully, he became more competent later.
The following relates to the Mitsuteru Yokoyama's anime adaptation:
Memetic Mutation: As mentioned in its article, Kaizo Traps are called "Koumei's Traps" (act."Kongming's Traps") after Zhuge Liang's style name (in the on'yomi reading). The ultimate origin of this name, though, is from this adaptation. Specifically, when Sima Yi heard about Zhuge's dealth:
Sima Yi: Wait, be calm; this may just be another of Kongming's traps'.