I'm assuming you're using lol-skill.net to make this comment. If so, you do realize lol-skill.net is just an algorithm and that their win percentages have no basis in reality? You shouldn't AFK just because a website tells you that you have a poor chance at winning. Also, was this ranked or norms? If it was norms then that's pretty normal because one of the big things lol-skill uses to calculate win percentages is familiarity with a champion and most people play champions they aren't familiar with in normals to practice.
If you're less than diamond then you are a shit player (myself included.) Doesn't give you any excuse to be toxic. If you got paired up with diamond players does that give them the right to shit on you because you're so much worse than them?