Hmmm. I've not watched Season 7 for a while, but I didn't particularly dislike Kennedy. I actually quite liked that she stood up to Buffy sometimes- I do agree, she was a little pushy in terms of her relationship to the Scooby gang, but, at the end of the day, she, along with all the other Potentials, was being asked to put her life on the line, and until Chosen, unlike Buffy, she had no superpowers to back her up. I think I'd be questioning the leadership if I thought it was a little dodgy. And she did seem to be a good fighter, putting effort in, helping train the other girls. And then, of course, she got together with Willow, and as has always seemed to happen on Buffy, once one of the core Scoobies gets a boy/girlfriend, they seem to kind of join the ranks themselves.
I do think she was a little bit... not badly written, but not really fleshed out. She was a Potential, she was a spunky, kind of arrogant, flirty forward type (I actually got the feeling she was written to be as different from Tara as possible, so that the audience wouldn't see her as just a Tara-clone, or Tara-Part-2). And that was pretty much it- there were so many Potentials, and so much Spuffy stuff in S7, that, like a lot of the characters, she lacked some character development. Unfortunately, unlike most of the other "main" characters, she didn't have prior character build-up to fall back on.
As far as her relationship with Willow goes... I wouldn't say it's too soon. I mean, after Tara dies (May 7th, according to her grave), and Willow goes Dark-Willow, she goes to the UK to study with the coven. I can't remember if it's ever specified how long for, but I imagine it was several months. When she returns to Sunnydale, some time passes before Giles arrives with the Potentials, and then later she hooks up with Kennedy, after initial reluctance. Bear in mind, that Oz left Willow (heartbroken because he was the love of her life at that point) about a quarter of the way into Season Four (after Halloween, so some point in November, presumably). I'm not sure at what point her and Tara become a "relationship", but even assuming that it's New Moon Rising, that can't be more than 6 months, right?