First off, I adore/worship/love Cordelia. That woman is amazing. So what if she isn't 'fun' anymore? She is a FIERCE, incredible hero, and keeping her as the standard shallow 'bitch' would be a huge disservice to the character, IMO.
Anyway. Onto Fred.
I can see why someone would dislike her from a narrative standpoint, since she fits quite neatly into the 'cute pixie nerd girl' trope, she doesn't do a whole lot, and she basically reduces Gunn and Wes into cavemen half the time. They were all good friends before Fred and Gunn got together, and they never quite returned to that. And I love Wes and Gunn, so I can see how that can be irritating.
But I think I would really like her in real life. In the narrative, she and a few other characters get pushed to the side because obviously there's a bigger picture there, but here's the thing about Fred as a person- she's smart as hell, compassionate, and brave. That's not always explored in the show, but it's what I believe is true about her personality. And I like people like that.
And while I totally understand character-hate (believe me, I've got some character-hate myself), I know there are a LOT of people out there who identify with Fred. Just the same way it is with a lot of characters. IDK, I'd say just be careful with all the hate...it can result in a lot of hurt feelings.