The Glorp Corp caused a major argument between the fandom on whether or not the members were going to be female or continue to be male. Eventually, information released with them as male.
Later, when Gobba went instead of Jawg with the rest of the tribe leaders in "Mixed Up Special", there was a giant rift caused in the fandom on whether Jawg or Gobba was the leader.
Every Knight Has Its Day got some negativity for not giving all three of the Series 7 tribes a major spotlight with one character, Tapsy, being completely absent from the episode. You can either see this a way to develop characters by not cramming too many major characters all in just one episode, or a lazy collaboration between the show and toy series.
The fandom loves Scorpi. And Nurp-Naut is climbing in the ranks just as high as him.
After Mixel Moon Madness, the Glowkies (Well, Globert and Vampos at least.) are very popular in the fandom.
Vaka-Waka is one, probably due to his funny German accent and having two heads with different personalities that makes him a funny and likable character.
Some of the background Mixels in Every Knight has its Day are this, particularly Booger, the Glowkie student, and the Teacher, who is the first female Mixel on the show.
Fanon: One of the most common fan ideas suggested is that the core members of the tribes are brothers, bar a couple. This comes from the fact that the 2015 waves of the established tribes are called the "cousins" of the original iterations of the tribes. note The only exception to this idea are the Medivals, who are canonically cousins with a pet, and the Nindjas, who are more often seen as a master and students relationship.
After the release of A Quest for The Lost Mixamajig's commercial, fan art depicting their fan characters holding the key of the Mixamajig and calling themselves "The Chosen One" started swarming in the fandom.
Chomly got six.
Are you deranged? I would love to have my ass kissed all day. note A rough EKHID storyboard of Mixadel saying this sparked much humor in the fandom.
Scorpi. The fandom loved him when only an image was shown, their love multiplied tenfold when his official information came out and he was shown in motion. Even characters in-show comment on his cuteness!
The Nurp side of Nurp-Naut qualifies as well, being a little baby voiced by an actual child.
Overshadowed by Controversy: Turg was originally called a "windowlicker" in the online store's description, which was an archaic, yet still offensive term in Britain. After a media frenzy, his bio was modified.
The Scrappy: To some people, the Fang Gang are this, though mostly Chomly (thanks to "Fang Gang Log Toss" where he abuses Flain and Slumbo just because they destroyed his log). Chomly's set is also one of the worst-selling ones.
Scrappy Mechanic: The teleporter beams in Calling All Mixels. One touch and you're zapped back to the start of the section. Naturally, the game has plenty of them, especially during timed levels.
In "Coconapple", Zorch mixes with Seismo in order to help him get his coconapple. The mix even strikes a heroic pose.
You'd Expect: The mix would fly heroically, getting the coconapple which Seismo wanted.
Instead: The mix flies uncontrollably like a plane about to crash, crashing through some homes and even getting stuck in a volcano.
As A Result: Seismo doesn't get the coconapple he wanted.
In the "Only You Can Stop Forest Freezes! Only You!" Segment of "Mixel Moon Madness", after Meltus accidentally melts the Camp Cube, Bernard gets some ice water to chill it back up, only for him to get chewed out by an angry Flamzer.
You'd Expect: Flamzer would snatch the ice water out of Bernard's hands, put the lid back on, and put the ice water away.
Instead: Flamzer refuses to put lid back on or even turn it upright, causing the ice water to pour onto the floor.
As A Result: Flamzer starts a forest freeze and nearly gets frozen solid if it wasn't for Bernard and Meltus's quick thinking.
Seismo in "Coconapple". Not only can he not reach his favorite snack, but Zorch makes him mix and then go flying, injuring himself in the process. And finally, he gets knocked unconscious by the coconapple he wanted to eat in the first place (which Zorch ends up getting).
Camillot for the majority of "Every Knight Has Its Day". It's obvious all he wants is friends, but his blunders only leave him ostracized and laughed at by everyone. It isn't until the climax that he's able to bring himself around.