I don't think she's very good, but she's a lot smarter than people think she is, and is deliberately cultivating her image the way she is. I think she's actually a very elaborate opiece of performance art, a prank on the entire music indsutry. She portrays herself as a vapid party girl, but really she's a very canny musician making full use of the ridiculous visual nature of modern music. See also Lady gaga, who is a very capable pianist with some very good songs under her belt, but got famous by being weird.
I don't like her music, but I like the fact that people do, because if I'm right (and I think I am), then anyone who likes her music is being deliberately set-up as the sort of party-pop-princess lover. Hopefully, Kesha will one day reveal the whole thing to have been a joke and everyone who ever liked her will be ridiculed, including by the artist herself.
A quite good example of this is the song Sylvia's Mother. It wasn't intended to be serious, but all the hippies thought it was, and when the band announced that it wasn't serious everyone looked like a prat for having called such a blatant parody good.