These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Drahmin is either a character with a lot of potential in the story or an annoying addition to the cast (see The Scrappy below).
Mavado has both fans and haters alike. Those who like him for being a Foil to Kano and being apparently the only decent Red Dragon of the cast (unlike his widely hated subordinate Hsu Hao) and those who hate him for apparently killing Kabal and stealing the latter's signature hookswords.
He also shows up in the second season of Mortal Kombat Legacy, again being the only character from anything beyond MK3 to make an appearance.
To a lesser extent, Li Mei. Her Plucky Girl attitude also helps.
Other popular debuting characters are Frost and Nitara.
Game Breaker: Certain characters had "impale" attacks with their weapons which would constantly sap life from the opponent until they keeled over. Granted, these attacks could be blocked, but there's a reason they never resurfaced in Deception and Armageddon.
Replacement Scrappy: Mavado is hated mainly for replacing Kabal, and to add an insult to this injury, apparently killed the latter and then stole his trademark hookswords. It earned the ire of Kabal's fans.
Rescued from the Scrappy Heap: Frost was hated when she was announced, giving that she would ruin Mortal Kombat forever, until the game is released and she's now a fan favorite by having a cool backstory, as well as her movelist being different from Sub-Zero.
John Vogel: We were fully prepared to say "Yes, [Jax killing him] really happened. He's dead. He's gone." But Mortal Kombat Armageddon is about bringing every character back to life, so Hsu Hao is back... much to my dismay.
Even years later, it seems that the team's dislike of Hsu Hao hasn't faded in the slightest. When Kano's design was revealed for X, a fan asked Ed Boon on Twitter if Hsu Hao would be "jealous of that chest piece". Ed's response?
Drahmin, although not to the extent of Hsu Hao, also earns a similar reaction from the fans. Ed Boon notes that he's one of his two least favorite characters (the other being Hsu Hao), and he's unpopular with fans due to how newcomer-unfriendly his playstyle is. The flies swarming him and constantly buzzing was a rather distracting nuisance.
And then we have his buddy Moloch. The developers even lampshaded this in Mortal Kombat X, by having Quan Chi carry around his head.
Mokap is a character most players either completely ignore or absolutely refuse to acknowledge, due to his design being the most unimaginative concept ever seen in a fighting game. He ended up making the ScrewAttack "Top Ten Worst Fighting Game Characters" list for that reason.