Man, there really is a stereotype of Master Yi players. I mean, I play the guy a whole lot, he's my...I think third-most-mastered champ behind Evelynn and Bard, but I always like to think I live up to my summoner name.
But...Based on the number of Yis I've seen who seriously don't get that he's a cleanup champ who absolutely has to wait for the other team to engage before he does or he's a dead man, I can kinda understand why. He's only good if you have the patience to not charge into a fight until it's good and well started. Otherwise, you're gonna have a bad time. He's also a really bad first pick because he's super-easy to counter-pick.
But I digress. I think there's a solid stereotype against Yi players for the same reason there is (or at least was) with Riven players: They gotta snowball. It's not optional. They just can't afford to be behind, or to mess up their first gank/all-in. So if they end up behind, they're going to spend most of the game miserably useless. And people, well, tend to give up easily, I think.