Non sequitur. I've heard people complain about Kristin Kreuk's Lana, but never on the basis of her hair color. (And for the record, some of us thought she did a great job as Lana. Just because the critics of a given thing make a lot of noise on the Internet, that doesn't mean theirs is the only point of view that exists.)
I honestly never really watched the show, all I know is whenever I happen to skim a discussion n the net or someone mentions the show, someone else almost invariably brings up how bad she was. Not that it really matters as I was actually being a little ironic and so help me if you come back with another hair splitting comment, I shall brain you.
As for Dunst, I never had much of a problem with her in the first two (only got about 20 mins into the third) but then since I'm not so familiar with the source material, I'm ill equipped to know if her characterisation was off. Literally, all I knew about her prior to watching the film was what I recall from the old 90's cartoon; which was basically 1) She was a model/actress 2) she spent a lot of time getting kidnapped and 3) She was a ginger.