Mr Rogers projected an air of genuine, unwavering... almost saintly pure-hearted decency, but when you look at the person behind the image... that's exactly what you find too., xkcd
This trope happens when a character is really sweet, polite, and very nice. Too nice.
We, modern viewers, know it is almost impossible for a fictional hero to remain something more than a Jerk with a Heart of Gold in this day and age, ifnotsomethingworse...
The Genre Savvy protagonist knows that the nice character can only have very low chances of being a hero and avoid brooding, and even less if they find themselves going so far as to be kind (and the audience wholeheartedly agrees) ! So nobody is really shocked when an investigation is launched and The Lancer starts Perp Sweating them out of nowhere. This kind of character must be hiding something, for old suspense's sake, at least a detail like being The Mole, a Gold Digger, orsomethingworse... except they aren't.
This happens when the story sets up suspicions of Bitch in Sheep's Clothing, but subverts it, either by making the character be the nice person they appear to be, either by revealing that they played another kind of nice than the usual one, but are ultimately just as decent as they appeared, if they weren't better.
Compare/contrast Jerk with a Heart of Jerk, when suspicitions of kindness about an usually Jerkass-like character are wrong.
Often delivers a lesson about how Evil Cannot Comprehend Good when the villain tries to Hannibal Lecture and thinks that he has found something evil in the hero, but there isn't anything evil to find and the hero proves them wrong.
Commander Vimes. Nobles assume (or just like to think) he's a jumped up copper who married his wife for money. Since he's a perspective character in several books, it's very clear that he loves his wife and hates the money.
Vimes's subordinate Captain Carrot also has people wonder if his Incorruptible Pure Pureness isn't just a front (it isn't, to the point where it's actually quite annoying to some characters).
Lord Vetinari, the Patrician of the city, often frustrates the ruling classes by honestly having no vices they can use to exploit him (although unlike Carrot, he's much more of a Magnificent Bastard, just not a selfish one).
Jane Fairfax from Emma is an Older Than Radio example. She was just a Shrinking Violet well-known as a sweet and innocent person secretely trying to keep her secret engagement, well, secret. Too bad a nosy, charming, and manipulative heroine came and almost ruined it all by beginning to flirt with her fiance and asking him what he thought she was hiding, because of her Inferiority Superiority Complex. Saidfiance felt "forced" to play along to keep the engagement a secret, began to give her whatever "leads" came to his mind, then to flirt back, then to toy with Jane's feelings. Guilty and disgusted, Jane took the very classy decision to try to break up with him and to resign herself to a life as a poor governess, but he finally went back to her.
Live Action Movies
Mansfield Park has Fanny Price, in-universe. She refuses to marry the young, rich and charming Henry Crawford, causing him a heartbreak and robbing Sir Thomas Bertram, the baronnet who brought her up since she was ten, from a lot of connections and honnors, something she was expected to do as a thank you. Fanny (whose thoughts are conveyed by the narrator without comment) starts wondering if she is mean, willfull and self-involved... but when reading what follows, it is very clear that she is just sane. Henry Crawford is a flirt who started a relationship with her engaged cousin, and she doesn't feel she should marry him to please him.
It happens to all new stepparents in family movies nowadays.
Who Framed Roger Rabbit has Baby Hermann. He is apparently a joyful and innocent baby who is very kind. But wait, he is actually a grumpy man who even has a love life... and is very kind.
Live Action TV
On Castle, Beckett's future Romantic False Lead Detective Demming was introduced as a Nice Guy, then came under suspicion by the team that he was a Dirty Cop and the Killer of the Week. Turned out that not only did he have an alibi, it consisted of coaching an underprivileged youth basketball league.
Sefa from Merlin. Gwen imagines her to be a traitor bent on revenge who wants to destroy Camelot and hides it behind a sweet composure, like her old friend Morgana. It is quickly made painfully clear that she just innocently gave information to her father, not imagining to which extent he would want to harm Camelot. Her motivation was simply to help her beloved father to get the respect he desserves, as she thinks everyone should, from a ruler who attacked their pacific people by mistake in the past. Even before knowing her father's plan, she feels sorry because the Queen (who, as far as she knows, would condemn her to death if she knew where she was born) was kind to her and encouraged her to pursue her crush, and let her eat her sit at her table.
To a minor, Not So Different-at-least-by-actions degree, in the third episode, when asked if she really spoiled the queen's life by her prince, Snow White bluntly answers she did, and spends the episodes stealing people. It is her only mean of survival, and hurting the Queen was unintentional. Then, in episode 16, she tries to kill the Queen, but she is saved from the spell which pushed her into this.
The Falaise castle in Normandy has an interactive tour guide where images of historical characters who owned the castle are projected on a wall to tell you the museum's version of their story. Almost all of them try to justify what they did but ultimately fail, from the last ones, like Alienor of Aquitaine (who here tries to claim that she believes everything that she caused happened Because Destiny Says So and because her family was money-crazy, but later gives up on the pretense and relishes in being The Vamp and an Evil Matriarch) and Emperess Matilda (this incarnation introduces herself as the only competent and sane member of the family but shows a terrifying Lack of Empathy because she is an Evil Overlord) to the first ones, William the Cute and PsychoBoisterous Bruiser and William's son The Resenter with No Sympathy. Then comes the matriarch of the family, Matilda of Flandres, who quietly explains how she ruled in the place of her husband in Normandy. She tells you how she disliked war, prefered to help the Church, and how guilty she felt when her marriage was declared invalid. And just when you expect a delicious, terrifying twist, this comes :
Matilda: And the archbishop of Cantorberry came on my burial ground, and he spoke to the crowd. And he said this, that William had defeated his enemies through war, whereas I defeated them with peace. He was wrong (looks up and really smiles for the first time)forIneverhadenemies.
In W.I.T.C.H, Will's would-be stepmother is not Nerissa in disguise. Just a geniuely nice person who fell in love with her father after he and he mother divorced. Granted, her name sounded a lot like Nerissa's, but she puts up with her stepdaughter's accusations of being an evil witch and lying about her name very well.
In Fairly OddParents, Timmy summons a popstar, hoping that he will be a Bitch in Sheep's Clothing and that forcing huim to marry his babysitter will be a way to torture her. However, he is very nice, and things go well.
xkcd presents Fred Rogers as a Sheep In Sheep's Clothing. It announces that a recording was found of him arguing with his wife. He acknowledges that he is sometimes angry at her, then tells her he is glad to have her !
Five hats means that five tropers think it is ready to publish.
You are saying that you think this draft is ready to be published. That means the description is not ambiguous,
it doesn't duplicate an existing trope, there are at least three examples, and the title makes sense.