Like Bang, it seems like half of the fanbase loves Carl, and the other half loathes him. The ones who hate him generally find him whiny, annoying, and irrelevant to the overall plot. Other people like him for his relatively unique, cool, and effective playstyle, puppetmaster aesthetic, stylish outfit and an interesting story overall. Also, the fact that some people (mainly the female parts of the fanbase) just want to take him home with them really helps his popularity.
Draco in Leather Pants: Jin gets along well with people, even Noel; he just doesn't say anything about his confusion. His jerkassness/partial insanity is a result of the Ikaruga War combined with him finding out that the brother he killed is still alive and remembering his past. This paid off. He's now one of the genuine good guys.
The vocal version of Gale -Reppu-, Bang Shishigami's 1st theme song was originally written and performed by a user from Nico Nico Douga, the song was so catchy and worthy of Bang's fame that it caught the Blaz Blue's official radio show (Buraji) attention. All the impending success of the song made Daisuke Ishiwatari invite Bang himself (Tsuyoshi Koyama) to perform the vocals for his 1st theme song, this of course with the very same lyrics the original writter envisioned. Oh, and in Guilty Gear x BlazBlue Live Music 2011, Tsuyoshi himself sings this same song with these same lyrics!
Even more impressive that Ragna and Jin have their share of fan vocals tied to their themes, still no word on Daisuke Ishiwatari promissing to have the same lyrics whenever he invites the voice actors to perform the official vocals.
Taokaka as well. In Relius' joke ending in Continuum Shift: Extend, she lampshades her popularity. (The kicker? She said this while in Hazama's body. Yeah, it's like that.)
Taokaka: Tao would never say that, 'cause Tao is adorable and friendly and a fan-favourite.
"Funny Aneurysm" Moment: Remember Tao's alternate ending from Calamity Trigger done in a cheesy trailer style for a non-existant "upcoming movie" that portrayed Litchi being an antagonist? Yeah...
Another one... Remember when Litchi suddenly sports devil horns when trying to keep both Tao and Carl quiet in her story? Well...
First of all, he's a Skill Gate Character who takes very little effort to win with in lower levels of play.
Second, his character design is one of the least anime-esque of all the Blazblue characters, as he bears some resemblance to some Marvel Comics characters like The Hulk, Beast, and Hellboy as a non Marvel example. As well as being more focused on what is viewed as awesome in Americans: MANLINESS, being the runner up in that department aside of Bang.
Lastly, the memes associated with him aren't as prevalent in the Japanese version, if they are present at all. The infamous GIGANTIC TAGER!!! meme in Japan just equals of his Japanese voice actor yelling "GAUNTLEEET HAAADDEEESS!"... the joke's still there, but less over the top and recognizable.
Ragna seems to be prone to this, like his younger brother/Rival, Jin. He has also been paired with almost every character from the series. What even adds to this is Rachel's Funny Ending. And just like Jin, this is not limited within the series. There're fanarts and fanfictions of Ragna being paired with Rita Mordio, Kohaku, Heart Aino, Lambdadelta, Souseiseki, Sol Badguy, Flandre Scarlet, Rumia, and more. And then, there's the official April Fool's BlazBlue X Arcana Heart - Crossover Flash Game hosted by the companies of both franchises. Where Ragna is on a date with Heart. Oooooohhh, boy...
Magnificent Bitch: Rachel. Truly proves to be the most powerful of the cast and shows what a piece of cake it is to use the Tsukuyomi unit to defend Kagutsuchi from Take-mikazuchi's threat, all without ever feeling like a Mary Sue or even a Villain Sue.
Memetic Mutation: Carl is known by many people as a pimp, mostly having to do with his almost all-purple wardrobe, complete with a pimp hat.
Narm: Before the start of Carl's fight with Nu-13 he says "I'll do my best, sis. I want to taste your cooking again." in the most dead-serious tone possible.
Porting Disaster: The PC release is apparently a bit buggy, and it uses Games For Windows Live for its online play.
The Steam release doesn't even have online play. And if you have people who can play with you locally? Getting local multiplayer to work requires setting up additional controllers in a separate launcher program, which isn't included with this version. So you have a fighting game with no multiplayer whatsoever.
If anyone hates Jin, it's because he is a "scrub" character. His abrasive and crazed personality is actually rather popular.
Rachel is hated for being god-tier.
Some fans hate Carl because he's very weak. These people tend to not use his drive ability properly, which requires Carl and Ada to attack in tandem. This is very visible with his Astral Finish, as the enemy can easily avoid it unless Carl does some serious work.
On the other side of the spectrum, some hate him because of the Clap Loop. Never mind the fact that it's extremely difficult to get the timing and inputs right, or even to set up, since most people with half a brain will want to stay the hell away from Nirvana.
Let us not talk about the counter hit -> juggle -> teleport Nirvana in -> claploop setup.
Unpopular Popular Character: Oddly enough, the minor characters from Ronin-Gai who follows Bang would avert this if it weren't, well, for their status as minor characters.