I love Dawn. Not ashamed. She's underrated. Not even that annoying. 😀
There are a few character sthat come to mind when one asks who the most hated character is... Such are Riley, Kennedy, Connor, etc., But it seems that the most universally hated is Dawn. I've even seen some call her the "Scrappy Doo" of Buffy.
I think what it is with dawn is that she has a lot of reasons not to like her and the reasons to dislike her far outnumber the reasons to like her. She could have been a good character if she was written better and I don't blame Michelle Trachtenberg for how Whedon made her... If anyone is to blame for Dawn's flaws, it's Whedon.
Speaking as someone who dislikes Dawn a lot, I'll say what most fans say about her, but I'll elaborate more and give credit where credit is due... Is she whiney and annoying? Absolutely. But in her defense, she does have a lot of reasons to be said way and kind of is justified.
She lost Joyce, Buffy, Tara, Giles, Anya, and then Willow went off the deep end and tried to destory the world... So shes lost so many familial figures, and to top it off, she, a typical 15 year old girl, finds out that she really isnt even a real person and that all her memories are fake and she probably wonders if she even has a soul and will experience an afterlife once her physical body dies... Also, the sheer awkwardness of being so young and potentially unwanted as a 15 year old among all the Scoobies, who are all in their twenties, must be really awkward and she probably feels like a baby among them. So yeah, she does have a lot of reasons to act the way she does. She's not completely unjustified, in her defense.
Another reason people really dislike her is Because her arrival essentially changed the memories of everyone and no one really remembers what truly happened in the first four seasons, which is really alarming and disappointing.
Also, Dawn being on the show made it so much more juvenile than it needed to be and essentially Buffy had to be her mom figure. Strapping Buffy down with a needy child was not a good way to run a show that was so phenomenal in it's first four seasons.
Would I have liked it better if Dawn jumped instead of Buffy at the end of season 5? Absolutely. I really dislike the direction of the show in seasons 6 and 7 and I think it would have been much better if it was done differently and Dawn was a one-season character. Many other fans share this view.
I will say though, she does show growth in season 7, but at that point, the show was really dragging on in a negative way and we had the nuisance of Kennedy to deal with...
Michelle Trachtenberg is a good actress and is a beautiful woman, but that doesn't change the way her character is. It's almost like Whedon and the writers wrote dawn in a way that they intended for the majority of the fandom to hate her.