I don't mind him being smarter than adults – I was smarter when I was younger than I am now. I don't mind him having the knowledge and ability of a person trained in the art – that also happens from time to time, but after hard work. I don't mind him solving certain problems they can't either – sometimes the problems can be out of the scope of their training and experience.
Problem was, he was a contradiction of traits and writers went out of their way to make him seem a genius. One moment he's acting stupid, immature and can't tie his own shoes, yet next moment he's solving a crisis. The crisis seems to be solvable by the crew, yet the crew can't, almost as if they wanted to leave to him to solve it. He's inexperienced, yet he's flawless when solving the hard critical problems. To make up for that, he creates a terrible mess when making a school experiment, under circumstances so exceptional that it still makes him look like a genius. His successes were never done in a believable way, and his inaneness on other matters was overdone in a way that made him look even less believable.
There's also how he was mindful and always almost stereotypically did the right thing, yet managed to go so wrong that another kid died when he mindlessly followed Locarno, and then proceeded to lie about it. At one point it felt like his successes and failures were pulled out randomly out of some book of contrived examples.
I am not sure, but I think that played quite a lot in what made him unlikeable. I was younger than him when I started hating him, I don't think I had such reflections at the time.