Granddaddy - a nickname given by most modelers to the RX-78 Gundam, due to being the very first one to bear the name.
Ped Comet - Another name for Char due to his rumored questionable relations towards underaged UC girls like Lalah and Quess (Puru is often roped into this group due to Super Robot Wars series coexistence hijinks).
Gruntmobile - The GM, The Federation's mass produced mobile suit in the UC continuity.
Gundam 0081 - The nickname given to the latest UC Gundam videogame project before its official name was known. The game was eventually released under the title Mobile Suit Gundam Battlefield Record UC 0081.
No Zaku - The MS-07B Gouf, after Ramba Ral's memetic declaration to Amuro that it is "no Zaku, boy! No Zaku!"
Student of the School of the Undefeated of the East - any character in any series (yes, even non-Gundam series) that does something suitably badass (i.e. manages to fight Humongous Mecha hand-to-hand, Nightmare of Nunnally!Zero and Alberto the Shockwave are the most notable) is often given this distinction (after Domon and Master Asia's reality-warping martial art feats).
The After Colony Era
The Perfect Soldier - Heero Yuy
The Bat-Gundam - The Deathscythe Hell, especially the Endless Waltz version, because of its resemblance to Batman.
The Druggies - Orga, Shani, and Clotho, the three Alliance Gundam pilots from late in the series; they can fight on the same level as genetically enhanced Coordinators, but require a special drug (with horrible withdrawal symptoms) to do so. Notable for being used as more of a term of affection than derision among the fandom, despite the name's negative connotations. The Japanese equivalent of this nickname is "Baka Trio", from the word for stupid or idiot.
The Japanese fans also refer to Sting, Stella, and Auel (the Alliance Gundam pilots from Destiny) as the New Baka Trio; American fans never really gave them a nickname.
In the wake of his death, Chairman Durandal acquired the nickname "Space Commie Naraku" due to his striking resemblance to the Big Bad of Inuyasha and the fact that the Destiny Plan is essentially Communism IN SPACE.
Bad/Evil Amuro - Ribbons Almark, Alejandro Corner's useful and traitorous servant, as a result of being voiced by Tōru Furuya, who played Amuro Ray from the original Mobile Suit Gundam, and there's also "Fucking Ribbons" after he reveals himself to be the true big bad and leaves Corner to die as well as many other bastard moves he does henceforth..
Prince Ali - Ali Al-Saachez, since his first name is a popular name for Arabian Nights princes, but most infamously the disguise used by Aladdin in the Disney version: Prince Ali. Thus, there has been some Memetic Mutation replacing Aladdin in his Prince guise with Ali Al-Saachez' face, and they even poke fun with his Image Song: "Prince Ali, Mighty Is He!"
Since his death, people have rewritten the meme to "Prince Ali, Mighty Was He." Rest in peace, O lovable bastard.
Johann Trinity, the eldest of the Gundam Thrones, is referred to as "Captain Hotpants" for being the leader of the trio and wearing very tight white shorts.
Setsuna F Seiei has...
Jihad-kun - particularly after his history with the terrorist organization KPSA was revealed.
King Arthur - Saji Crossroad, according to Soul Bro Ryu of the Gundamn! podcast.
Papa Bear - Sergei Smirnov, due to his in-series nickname "Wild Bear of Russia". Sometimes called 'Pedo Bear' due to his interactions with teenaged Super Soldier Soma Peries... even though his feelings were obviously fatherly.
After he dies, the nickname 'Pedo Bear' is inherited to his now much-reviled son Andrei Smirnov for the latter's affection towards a younger Louise Halevy.
Lecharous Lyle - after events in episodes 3 and 4 in season 2.
Doublelujah - (H)Allelujah, due to both personalities being active at once during (H)Alle's Crowning Moment Of Awesome in the first season's finale.
The Red Ranger - Anew Returner. When the CB members got color-coded outfits in 00 season 2, fans wondered where the "red ranger" was. One eventually showed up. However it was Soma Peries/Mary Parfacy who dons a Red pilot suit and mecha in batttle.
The A-LAWS are less affectionately called as the A-HOLES by fans.
Also known as A-LOLS.
The name 'Bushido Bob' was given to Graham Aker in the mid-series break due to the mask he was seen wearing in the previews for season 2. The authors thought it was either cool or funny because he returned with the name of 'Mr Bushido', with the note "That's what they decided to call me." (It's all your fault!)
And because his new mechas, the Masurao, later upgraded to Susanowo, can use the Trans-Am system, it's jokingly called the Trans-GrahAm by fans. (Or more affectionately Trans-GARHam.)
One fan-dub of The Movie even uses this when he activates it to help Setsuna against the ELS blocking the way.
Some less affectionate fans call him "Flag fucker" for his... overly affectionate treatment of the various mecha (mostly Exia).
Speaking of his Flags, his last two mechs in season two have been called Shin Musha Flags due to their Samurai-based design.
And among those less than fond of his S2 image change described above he's known as "Mr. Bullshito".
Other fans have also called him Graham Crackers◊ due to the letters of his surname fitting in the second word.
GN Guncannon - The "Reborns Cannon" portion of Ribbons' "Reborns Gundam".
Colonel Fatass - nickname given to Arthur Goodman, the fat A-LAWS commander.
An alternative is 'Captain KFC'.
According to the commentary from Ep1 of Season 2, the crew referred to him as "Baker-San" after a comment by one of the cast.
The ESF president from season 2 was given the nickname "00bama", for looking very similar.
SoMarie -Soma Peries/Marie Parfacy, a portmanteau nickname.
Darth Louise - Louise Halevy in season 2, due to having a Red Right Hand (well... left hand in her case, but still) quantum brainwaves, mostly answering only to Ribbons, and other similarities between her and Lord Vader.
Gundam DD - The entire series in a whole for having many busty women.
Nena 2 - Meena Carmine, for having her name remain unrevealed for serveral months and for being Nena's Identical Stranger.
Corner's Counterattack - The hilarious film within a film shown at the beginning of Gundam 00: A Wakening of the Trailblazer. The fact that the asteroid base shown bears an uncanny resemblance to Axis does not help.
China Bitch - Wang Liu Mei, the Chinese Smug Snake who was quite unpleasant and whose attitude throughout the series was "I want the world to change, but I don't care who does it." Occasionally called China Tits because, like almost every other female in the series, she was large-breasted.
Super Pirate: Captain Ash of the Bisidian pirates, because he's a pirateand Asemu Asuno.
Kira Asuno/Kio Yamato: Kio, after adopting Kira's anti-killing mentality.
Flit's worsening attitude towards the Vagans over Asemu and Kio's arcs has earned him the title Flitler.
And/or Grandpa Genocide in the Kio Arc.
Fans have dubbed "Flitoshiki"◊ an edited version of Flit's statue at the end of the series, complete with the kanji symbol "100" - supposedly a Shout-Out to the MSN-0100 Hyaku-Shiki from Zeta
Gundam Build Fighters
Gundam Boy Friends, since the show's title abbreviates to BF. Then there's the fact that there are abundance of shipping material, both the Ho Yay and straight variety.
Sazaki is often called Gyandere, due to his obsession with Sei and his preference of Gunpla.
Rinko is called Mama-n, the suffix ~n is used in Japan as to point out cuteness aloud.
Also it sounds the same like the Japanese slang Maman, who is the equivalent there of the phrase MILF.
The moment Sengoku Astray appear fully as model kit, it earned the title Weeaboo Astray, being overtly Japanophile all around (even the name Gundam is written in Kanji)
The alliance of pilots who attack Fellini in episode 11 was quickly dubbed "Team NTR", since their entire motivation was revenge for Fellini stealing their girlfriends (though he insists that they came on to him).
Rainer is known as "Butt-hurt NTR guy" in Japan. Oh, and he lost the qualifier too.