To me, Cyndi is a "love to hate" character because damn it, she's just so brilliant in her manipulativeness. Tharqa, Bob and Elmer, on the other hand, have no subtlety or cleverness or anything other than basic scumminess about them; these I simply hate/dislike without even the most grudging bit of respect.
Karen, as far as my reactions go, is somewhere in the middle. On the one hand, her villainy showed a considerable degree of planning, forethought and leadership skills, especially at the beginning of "The Popsicle War," so to me she doesn't quite fit into the "simply hate" category. On the other hand, as the War progressed she increasingly found herself caught off-guard and panicked by things that didn't follow her "script," and thus had to rely on her co-conspirators for direction; also, her insecurities eventually led her into overkill (like kicking Aggie when she was down, thus giving Aggie and Penny further impetus to team up against her) and into going after Duane, who wasn't remotely a threat to her, just for the purpose of keeping her clique together, and thus apart from the spiked popsicles her downfall was her own fault. So she doesn't fit the "love to hate because she's so clever" category either.
Then there's the matter of sympathetic aspects, actions or backstories versus the lack of them, but that's a whole other scale of measurement. (One where I'd have to group Cyndi along with the dimbulb thuggish villains because we have yet to see a single aspect of her that's sympathetic in any way.)