Since a lot of fans hate Mineta, he often gets portrayed as even more unlikable, idiotic, incompetent and perverted than what he actually is. He is already regarded as a Nominal Hero among the fanbase, so if there's a character who needs to play the hateable antagonist role, it'll often be him. In extreme cases, he is portrayed as an actual rapist. He is indeed one of the characters from Class 1-A that needs the most Character Development and training, but fanworks tend to make him a lost cause. Fanfics with Ho Yay pairings have gone as far as to portray him as a homophobe. In fact, "Homophobic Mineta Minoru" is a dedicated tag on Archive of Our Own. Some works also portray him as a transphobe, and some even combine the two.
Bakugo gets this, to a certain degree, given how cruel he was earlier in the series, but sometimes his cruelty is overamplified, despite the character development he has been put through as the manga progressed and his haters tend to pretend he has no redeeming qualities. His violent side is often exaggerated, with some works even having him try to kill Midoriya out of rage.
On the flipside, some fans who want Bakugo to look better than he was will try to demonize Midoriya somehow, a common one being to portray him as an annoying or even creepy Stalker without a Crush, justifying Bakugo's actions by saying he wouldn't have had to bully Deku if the latter just left him alone — barring the obviously disturbing implications there, this also undermines the aforementioned character development on Bakugo's part by presenting him as blameless and sympathetic from the start and not as a flawed person who needed to learn to be better.
As mentioned in the Fandom-Specific Plot page, it's common for Midoriya to be a villain, generally under All For One, though this is less of a hate towards the character and more due to the very concept being interesting, as well as his origin story being a tragic one, almost to the point of bordering on Tragic Villain.
Monoma is a Jerkass in-universe, but many fan works exaggerate this, making him more cruel than he actually is. In some cases, he’s simply portrayed as a one-dimensional Butt-Monkey, but other works go as far as to turn him into The Sociopath, having him do extreme things like mocking people for mourning another person’s death. His Hidden Depths are rarely shown, and most of the time, he is portrayed as the villain of the story, bullying either Midoriya or Bakugo or anyone in Class 1-A without an ounce of sympathy.
Shinsou is usually treated as an Ensemble Dark Horse, but there is a minor backlash among some fans to his many appearances in fan works. These fans exaggerate any of Shinsou's negative traits and portray him as an entitled, selfish brat who undeservedly wants to be a hero but is unwilling to put in any effort.
Iida is overly strict in canon and has some moments where he is a jerk, but fanworks sometimes exaggerate these traits to make him a massive Jerkass who is prejudiced against quirkless people or those with "villainous" quirks.
Ibara's religious views are often exaggerated by fans, turning her into a self righteous and homophobic fundamentalist.
UA High Faculty
While he might be an overly harsh teacher in canon, Aizawa is sometimes portrayed as even worse by the fandom. In many works, he punishes students for no reason, basically bullying and torturing them. Other times he discriminates against quirkless people, and is generally apathetic to the existence of his students.
Nezu is rather manipulative and secretive, but fan works sometimes exaggerate all these traits to make him into a Manipulative Bastard that will achieve his goals by any means.
Vlad King is often a go to pick to make a member of UA's faculty into an asshole or Straw Character, as he is a bit of a smug jerk in canon and has been given less focus than any other teacher. Fan works sometimes portray him as extremely mean or even a traitor to UA. To a lesser extent, Snipe can also sometimes be portrayed as a huge jerk in fan works.
This happens to nearly all of 1-A and the teachers in the Fandom-Specific Plot where Izuku is accused of being a traitor due to circumstantial evidence. Most, if not all of the students and teachers turn on Izuku instantly, depsite all of the time they've spent together and no matter how flimsy the evidence. Aizawa is usually the harshest towards Izuku in these fics, sometimes even torturing him to admit his guilt.
Interestingly, this often happens to All Might of all characters. Despite being the Big Good of the story, "All Might Bashing" is a surprisingly common tag in fanfics that portray him as a Jerkass buffoon and exaggerates any of his flaws. He is often portrayed as idiotic, arrogant, and selfish, a far cry from his canon portrayal. Any flaw or negative moment of his character, such as telling Izuku that he can't be a hero like him without a Quirk, is exaggerated to represent his entire character.
Despite being the first to bring up the traitor possibility, Present Mic is probably second only to All Might in the number of fics that make him a traitor or villain. Part of it is probably inspired by the fact that the man does have a vicious streak if you mess with someone he cares about, particularly Aizawa. He’s also highly intelligent in canon despite rarely showing it clearly. He can end up a sadistic, cruel, scheming jerk in some fics.
Pro Heroes
Endeavor is an abusive father in canon, but some fan works ignore any of his positive traits or his redemption arc to make him an irredeemable monster. Some fanfics have him be perfectly fine with causing the deaths of innocent people.
Sir Nighteye is canonically a huge jerk to Izuku, but fan works exaggerate this by not giving him Character Development like in canon where he eventually learns to respect Izuku, and instead have him remain extremely rude to him.
There's no denying Nana Shimura placing her son in foster care ended up backfiring terribly, but it's still clear she did it with good intentions in an earnest attempt to protect him, not to mention it obviously pained her to do so. Doesn't stop some fans from declaring her a horrible and neglectful mother who abandoned her child for selfish reasons despite evidence to the contrary.
Uwabami was undoubtedly a poor internship provider for Momo and Kendo, but in canon is still shown to be a legitimate hero during the Raid arc and while Momo was deeply in denial at the time, being involved in commercials is still a part of being a hero, especially one who doubles as a model. However this doubling as a model is all many fanfics focus on, where Uwabami is part of the list of 'fake' heroes that not just Stain goes after with a bloody vengeance, but others will consider a fake hero, completely ignoring her rescue efforts in the Raid Arc. Even without someone trying to kill or discredit her, she'll typically have her modeling career be used against her during discussions of internships to get Momo and Kendo to go to someone else in a odd mixture between this and a Fix Fic (as the two were undoubtedly unhappy with their choice, but there will often be an air of Uwabami being a bad hero in general when this is done).
Hawks gets this treatment as well sometimes, mainly in regards to his relationship with Twice. There are those who feel that his being a False Friend to Twice and his subsequent Shoot the Dog moment once he's revealed as The Mole and kills Twice is unforgivable. Nevermind that Twice, while highly sympathetic, was one of the greatest threats in All for One's arsenal, and both A) spurned Hawks' ill-fated attempts to win him over and B) was about to start slaughtering more heroes. Instead of a case of tragic circumstances making a happy outcome impossible and Hawks having to make a tough split-second decision to save more lives, he's seen as a hypocritical, remorseless scumbag. This increased when Hawks chose to stand with Endeavor after the latter's abusive past got exposed, which lead to fans accusing him being an abuse apologist and victim blamer, even though he did so more out of an acknowledgment Endeavor is seeking to atone for the abuse than any endorsement or denial of said abuse.
Main Character's Families
Hisashi Midoriya, Izuku's father, is frequently portrayed as being as abusive and neglectful as Endeavor if not an outright villain despite not having a single onscreen appearance, usually as part of a Die for Our Ship to push All Might or Aizawa towards Inko or to give Izuku a tragic backstory. This usually has him disown Izuku for being Quickless, or abandon the family for Izuku being Quirkless, when by all indications he's simply working overseas. Some fics even have him be the same person as All For One, which often tend to be on the nicer end of this treatment for Hisashi in a case of Jerks Are Worse Than Villains where fans see a supervillain active in his son's life as an antagonist as preferable to a regular man inactive in the same son's life for any reason.
There are fans who claim Rei is just as much of an Abusive Parent as Endeavor since she scarred Shoto's face with boiling water. This ignores that it was a Moment of Weakness (as opposed to part of a pattern) she immediately regretted that came as a result of her husband's abuse hurting her mental state. Another claim is that she was willfully complicit in his abuse of their children and did nothing to stop it. While Rei was a Useless Bystander Parent to an extent, it was shown that she did try to protect Shoto from Endeavor, but his abuse also extended to her and made it harder for her to fight back.
Bakugo's parents, Mitsuki and Masaru, are sometimes portrayed as abusive to Endeavor's degree, with Mitsuki being actively abusive and Masaru being a Useless Bystander Parent. This is due to some fans not taking lightly how Mitsuki has taken to physically disciplining her son after his kidnapping. Some of Bakugo's fans also believe that it is this supposed abuse that made Bakugo such an angered and hateful person to begin with (e.g. when he proposes forcing the Masegaki kids into submission, he remarks that was how he was raised), even though the manga went to lengths to explain that it had nothing to do with his parents, aside from the fact that they failed to notice their kid was becoming aJerkass.
Endeavor's Draco in Leather Pants status has caused some fans to demonize his family. Since Natsuo in particular adamantly refuses to forgive Endeavor, he is a frequent target and gets demonized into an Ungrateful Bastard with an Irrational Hatred for his father, some saying he was a Jerkass to not forgive him even though Endeavor himself acknowledged Natsuo does not owe him forgiveness.
On the flip side, those who dislike Endeavor tend to give Fuyumi this treatment since she's the most forgiving towards him. This can get exaggerated into denying or defending her father's abuse and dismissing her brothers' trauma.
Even Inko isn't safe from this trope, despite being a loving mother and a complete sweetheart. It doesn't seem to be out of dislike for her character personally, but some fanfics will turn her into an abusive parent to make Izuku's life more depressing for whatever reason.
Organizations
The Meta Liberation Army are villains in canon primary defined by the idea of using their powers without legal restriction, but in fanfics they have quickly become a go-to group for Quirkless discrimination and mistreatment. On top of this aspect of canon being greatly amplified by fanfics than what canon ever showed, this entire concept applies to the Meta Liberation Army entirely from one line by Geten about society being measured by the strength of one's Meta ability, taking a one off line from a non-leader of the group and making the entire organization and linked groups hostile to the Quirkless in everything from being a hero to basic human rights.
From what little we saw in the manga, Aldera Junior High is just an average Japanese middle school that Izuku and Bakugo went to before attenting UA. And while Izuku being bullied for being Quirkless is canon, the entirety of said bullying came from Bakugo and his cronies. However, in many fanfics it's not just Bakugo, but the entire student body that relentlessly bullies and discriminates Izuku, with the teachers encouraging it at best or joining in the bullying themselves at worst, on top of punishing and blaming Izuku for said bullying. Some fanworks make the school even worse by having it secretly being run by by the Meta Liberation Army in order to indoctrinate youths in their ideology.
will turn her into an abusive parent to make Izuku's life more depressing for whatever reason.