Trivialisation (Brit.) or trivialization (Amer.) is the act of making a metaphorical molehill out of a metaphorical mountain.
Trivialisation of rape is a charge, as is trivialisation of historical atrocities. This is the reason the use of certain canon monsters and historical atrocities as assassination methods is forbidden. There are many other trivialisations that are also charges. They can be split into two categories: trivialisation of canon events, and trivialisation of uncanon/fanfic events.
Trivialisation of Canon
Heavy things happen in canon. Isildur slipped the Ring on and fled to his doom. Nearly all of the Jedi were killed in the Great Purges. Dumbledore confronted Grindelwald in 1945. Denying that these things happened in their respective universes is a charge. Similarly, portraying in-universe cataclysms (wars, for instance) as trivial and unimportant is also a charge.
Canon characters can also have some pretty serious personality traits, such as insanity or violent tendencies, that can't be brushed aside without consequence. However, many badficcers exercise motives unrelated to proper characterization when writing fanfic, most commonly when shipping. This is not to say that writing a romance around an evil character can't be done, but an evil character in an honest romance must have a LOT of well-executed and believable character development to allow it — something that many Suethors and badslashers don't bother with. Thus, all of their past misdeeds are often written off as the character being 'misunderstood.' Ignoring a character's negative traits in order to facilitate a pet ship — or anything else — is a charge.
Trivialisation in-fic is what happens when a Mary Sue or possessed canon begins to act as if the world's rules don't apply or are unimportant to them. Unfortunately, there are an astounding number of fics and Mary Sues that invoke:
- Trivialisation of cruelty. Torture is scarring to the victim. It is different from BDSM, which in real life is safe, sane, and consensual.
- Trivialisation of weapons and violence. Guns are weapons: if a character carries one, it is a tool for sport or it is used to kill others. Same goes with all weapons: they are worthy of respect, and anybody who carries them is now lethal, not merely 'cool.' Killing people or being dangerous does not make a character instantly cooler, and killing people for the hell of it is a mark of sociopath tendencies rather than any positive quality. If an otherwise 'good' character behaves this way without some compelling reason, it is bad characterization.
- Trivialisation of self-injury. Cutting is self-mutilation. Suicide is a mark of extreme depression or REAL insanity. Neither are beautiful or romantic.
- Trivialisation of crimes. Murder is a crime, and is not ignored by many canon authorities. So is theft, destruction of property, vandalism, etc. Policemen or other peacekeeping/law enforcement forces actively ignoring a known criminal without a reason may indicate that the writer doesn't think the society and laws of the canon universe are important to that character, which is probably a Mary Sue.
- Trivialisation of romance/marriage/social institutions. It is a charge to go to great lengths to describe the great romance of Mary Sue and then make it out to be small potatoes. Important to this idea is that the characters involved AREN'T fickle, disloyal, polygamous, or otherwise established to have this sort of behavior as part of their characterisation.
- Trivialisation of injuries. Many Mary Sues get wounded and then for the next few scenes seem totally fine — their injuries only apply when it would be the most dramatic or garner the most sympathy.
Some humour fics actively choose to ignore some things in order to make a universe funny. For example, Darth Vader trying to yell at tech support or order a pizza is hilarious in good execution — but may dismiss the fact that he's killed hundreds. If this is truly funny instead of bad, a fic may get away with trivializing certain things without a charge. Then again, fics that succeed at this are rarely badfic, but rather the rare good humorfic. But other things, such as real life tragedies and holocausts (especially ones within human memory or a generation from it), are never funny even in the blackest of black humour.
In-Universe Rules or a Masquerade
Some settings actually have an amount of trivialisation built in. In tons of Looney Tunes cartoons, for instance, dropping an anvil on a character does nothing more than squish them like an accordion, after which they flop around making funny noises for a while. There are even cases of 'rape' comedy, like the Looney Tunes character Pepe le Pew. The in-universe rules of the cartoon setting allows things like this to happen for the sake of humour, even though in the real world they would not be funny at all because the rules here are different.
Also, settings that involve some kind of mass glamour, weirdness censor, or masquerade have magical or scientific reasons for why people may not notice dragons walking around, and inexplicably every time Godzilla destroys Tokyo it somehow gets rebuilt without any economic or social strains. These 'trivialisations' are built into the rules of the canon they take place in, and are not to be counted as charges. If Godzilla destroyed the real Tokyo, however....