This author made a pretty good point about Yui-bashing…

I think that the kindest word the critics have used to describe Yui is “airhead” – and not in a particularly affective tone. I mean, this is so wrong. What do they expect? Why can’t a show about friendship be as valid as a show about how power oppresses the individual? What do they want from Yui? Why does everything has to be intellectual? (as well as having a tyrannical link to real life issues, but that’s for another day) I mean, Yui may not be the sharpest tool in the box, but we can’t deny something. She’s a f**king great person. She’s really kind-hearted and caring. Sweet and friendly. Unpretentious and easy-going . Unlike many a critic. But this fact isn’t of course worth their attention.
– Sergio – Source

Keep in mind that it is a show and you shouldn’t get so serious about it. Not only that, I’m sure that there are other people that are worst off then Yui such as people who have learning disabilities. Yui is just a ditzy type of girl, this doesn’t mean she is dumb. What if you were in her shoes and people calling you “airhead,” would you like that? I wouldn’t think so…

Bullying is just wrong. I have dealt with bullying with my life and know how bad it is… Just do a favor, don’t do it or better yet… LEAVE HER ALONE!