I think I know why people complain more about Yasuo than about Jax or Mundo, for example

Because they see waaaaaaaay more times on their matches, when compared to champions like Jax, Mundo, Brand, etc

In addition, Yasuo has always been popular. The other aforementioned champions only recently became popular.

The problem is that Yasuo is such an incredibly popular champion, with many people that play and main him. Because he is so constantly used, and due to the fact that he is strong and annoying, people tend to complain about him more, and hate him more. Even if he may not be as strong as other champions.

I honestly don't know how to explain why he is so popular. It may have to do with a couple of things:

1. His theme. He is a samurai. Samurais are perceived as "badass" and honored warriors, so everyone wants to be a samurai in a game - in addition, they have to do with the Japan, a place where there are... animes. And there are many anime fans on our community.

2. His kit. He is manaless, is AD, scales very well with crits, can snowball very easily, and has incredible potential depending of your team composition.

3. His ability to counter most popular mages very easily.

4. He is a melee carry. People tend to like melee carrys because, at least at low elos, they have the best chance to carry by themselves, because generally you can't rely on your team to carry at SoloQ.

5. Yasuo has the ability to make flashy, fantastic plays (aka #LC$BIGPLAYS) especially when combined with other popular champions such as Lee Sin and Orianna.