Trivialisation

Trivialization is the act of making a (metaphorical) molehill out of a (metaphorical) mountain.

Trivialization of rape is a charge, as is trivialization of historical atrocities. The latter is the reason the use of historical atrocities as an assassination method is forbidden. There are many trivializations that are also charges, but specific to canons. However they can be split into two categories: Trivialization of canon events, and trivialization of uncanon/fanfic events.

Trivialization 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 Grindenwald in 1945. It is impossible to deny these things happened in their respective universes.

However, it is a fact that many badficcers have ulterior motives when writing fanfic. Although this is usually pairing-oriented, it is not always. For example, Evil Canon Characters 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 written off as the character being 'poor and misunderstood.' This is just as much a charge as trivialization of rape.

Similarly, portraying in-universe cataclysms as trivial and unimportant (wars, for instance) is also a charge.

Trivialization In-Fic
It is a sad reality that truly terrible badficcers barely understand what they are writing. For example, making a character that is a professional hitman implies that it is their job to murder people, and not at all cool or good in concept. It's the execution of said concept that makes the character -- not the job-- cool. Trivialization in-fic universe is what happens when a Mary Sue or possessed Canon begins to act as if the world's rules around them don't apply or are unimportant. Unfortunately, there are an astounding amount of fics and Mary Sues that invoke:
 * Trivialization of cruelty. Torture is scarring to the victim. It is different from BDSM, which in real life is safe, sane, and consensual.
 * Trivialization of weapons and violence. Guns are weapons: If a character carries one, it is a tool for sport or 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 sociopathic tendencies rather than any positive quality. If an otherwise 'good' character behaves this way, it is not only bad characterization but also somewhat concerning for the author of said work who thinks this is normal behavior.
 * Trivialization of self-harm. Cutting is self-mutilation. Suicide is a mark of extreme depression or REAL insanity. Neither are beautiful or romantic.
 * Trivialization of crimes. Murder is a crime, and is not ignored by many canon authorities. So is theft, destruction of property, vandalism, etc. etc. Policemen or other peacekeeping/law enforcement forces actively ignoring a Mary Sue without a reason may indicate that the writer doesn't think the society and laws of the canon universe are important
 * Trivialization 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 characterization.
 * Trivialization 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.

Humorfic
Some humor 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. In tons of Loony Toon cartoons, 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. 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 to do this are rarely badfic, but rather the rare good humorfic. There are even cases of 'rape' comedy, like the Loony Toons character Pepe le Pew. 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 humor.

In-Universe Rules or a Masquerade
Some settings actually have some amount of trivialization built-in. 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 'trivializations' 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...