Zexal is not that bad. People just hated on it because it had a slow start. At first it seems childish and derivative of the original. By the time we get to arcs like World Duel Carnival and Barian, it’s much different. The numbers club don’t prove all that important any more than Tea and Duke Devlin were in original Yugioh. That’s fine; because Shark and Kite are better uses of screen time than Cat Cathy and Kronk from da Bronx…
I didn't care much for the World Duel Carnival arc. From what I remember at least, it started off pretty poorly with most of Yuma's duels being filler and the tournament not properly connecting with the actual conflict with Tron and his sons until Yuma dueled III, which was the last qualifying match for him to make it into the World Duel Carnival. The later duels were better, particularly Shark vs. IV, Yuma vs. Shark and Yuma vs. Tron, but I would still say it's too little too late. Not to mention Dr. Faker is by far the worst villain in the entire franchise. Everything about him is just surrounded by terrible writing.
The second season did feel like an noticeable improvement, or at the very least it was more interesting and engaging than most of the first season. The Barians were more memorable antagonists than Dr. Faker, even though Tron is probably still my favorite Zexal antagonist. The last two arcs of the series were genuinely good. The Legendary Numbers arc was my favorite and while I had some more issues with the Barian Invasion arc by comparison, it was still engaging with some good duels and a pretty satisfying ending. The second season doesn't fix all of my problems with Zexal. There were still plenty of bad/sloppy writing and decisions that I didn't like, but compared to the first half of the series, it was more interesting.
I'd give Tea a bit more credit here. She wasn't unimportant in DM. She played a key role in finding Atem's name. Tea is basically Kotori done right, or at least done better. She is the token girl of the group, but she actually has a personality, her own desires and has connections with other characters besides Yugi. Kotori is basically Yuma's shadow cheerleader. She rarely interacts with any other character, her whole world revolves around Yuma and she has no dreams. Tea wasn't the odd one out as the only non-duelist in the main cast either. Kotori asks Rio once to teach her how to duel and her only duel in the series was when she was brainwashed for an episode. There is no good reason why Kotori had to not be a duelist, especially when every other character in the cast is. It isn't surprising in a way when the head writer for Zexal is not known for writing female characters particularly well, but it is still pretty disappointing.
While I agree that Shark and Kaito are better characters to focus on than the Numbers Club, I think that's missing the forest for the trees. The Numbers Club shouldn't have even been a thing to begin with. They're all boring characters and you can easily remove three of them without changing anything. As much as I don't like Kotori, I don't think she is as easily removable, if only because she's in every single episode for some insane reason, and Tetsuo at least saved Yuma from Number 96 early on in the series. If Yuma's friend group was just Kotori and Tetsuo, they could have potentially been able to create a chemistry with that friend group. I don't think it would have drastically improved their personalities to make them good characters, but trimming out the useless unnecessary characters in Yuma's school friend group would have been better. The Numbers Club never felt like a group of friends, but a group with one friend in common and even then, Yuma barely interacted with anyone who wasn't Kotori or Tetsuo anyway.