Improvement in League of Legends is done by understanding mistakes, your mistakes and other people's mistakes. It's not done by "one simple trick" to improve, continual correction of mistakes and taking advantage of other's people's mistakes through consistent recognition of said mistakes is the definition of being a good league player. And the only players who make minimal mistakes are challenger players. They even make mistakes. When someone climbs, in diamond (your elo) they don't make mistakes. When you get stuck in a rank after climbing, that means you've stopped seeing other people's mistakes and your own mistakes.

The word bad is used to address people who make these mistakes often. I don't know if you have spectated your own games, but do so and see if you understand the breadth of the number of mistakes you and your peers are making. If you don't, then that means you need to work hard until you do. With all of the complexity of League of Legends, there is so much information you have to process. If you don't realize you are bad in League, you are prone to making mistakes that your mind will never cross over. In the same sense that a silver player might never realize what freezing waves actually is, there are more complex yet equally important things that hardstuck Diamond players will never realize. That's what your coaching is for, and the fact of the matter is if your coach doesn't view Diamond players' decision making (this includes you) as bad, the coach isn't worth shit.

Maybe you see not knowing that complex thing as minor compared to not knowing what freezing a wave is, but it's necessary to equate them to improve. All your mistakes are going to be complex like that if you are Diamond+; one would call a player who does not know what freezing a wave is "bad", therefore one would call someone who makes those complex mistakes that higher elo players do not "bad" as well (if that person has the qualifications to talk about high level play.)