It's possible. Her mother Pebe Sebert is a performer and used to tour w/ Mick Jagger and the Rolling Stones. Pebe does not know who Kesha's father is, therefore leading to speculation that Mick Jagger could quite possibly be him. I'm not implying Pebe is a slut, but I have no doubt in my mind that she slept around w/ the band.
Reference: Kesha's official myspace.