You're level 13, that's when people start learning a bit more about the game.
Generally, someone with absolutely no MOBA experience will get wrecked by Yi. Everybody at your MMR (you might get a few games with smurfs, but eventually that stops) only has access to cheap champions and free to play ones for the week, so people will pick up Yi as their go-to assassin.
No comment on Poppy, since I'm... not a level 1-20 player anymore, so I can't comment on her new-player experience.
Amumu is a tank, and a jungler at that, so he's not appealing to new players who can't even jungle yet (Amumu isn't exactly the best at laning).
Annie is appealing to mage players, obviously.
Ashe is kind of complicated since the rework, but new players don't know how to optimize their cs and positioning and whatnot, plus the power curve is heavily delayed by the lack of gold mechanics. Assassins give power early and mid game, and they give it hard. It's much more direct and now than an ADC is, who's more quadratic with some exceptions (Graves for one, but he's expensive for a new player. You probably see a lot of Miss Fortunes, however.).
Anyway, as you gain a few levels and champions you learn to play against Yi. He will wreck you. A lot. Happens to everyone. But eventually you'll get sick and tired of it and force yourself to learn. You'll ward that one bush he always runs through. You'll pick up Morgana and learn the difference between hard and soft cc. You'll learn tricks to make Yi so useless that you'll reach an MMR where you never see him played again.
So, uh, yeah. People hate him early, but it's a necessary hate to learn how to play the game.