I don't really like Yona either...I agree that her voice is really annoying. Also, she seems pretty much meaningless to the plot. As far as I can remember, she only appears three times.
Firstly, when the Watershippers try to convince Vervain is insane and she sings the slug-a-moon song. That was better in the book. Speaking of which, Yona is a mythological character in the El-ahrairah stories in the book, and here she interacts with the WD rabbits. I don't like this change...
Then, there's a brief appearance in the Christmas special. She goes to sleep between two rabbits that were never seen before, and we will never see again. The whole scene feels incredibly pointless.
And finally, in season 3 she tells Hannah about the magic. I hate magic This doesn't really make any sense, since Hannah said in season 2 she didn't really like Yona. I think it possibly would have worked better if it had been Tassel who had told her about the Hedgewizard or just skip magic entirely.