I always thought Karen was great. I will try my best English major impersonation and say people don't really like or understand Karen because she's not the main character, the main character in the anime is Hikari, and the story is mostly being told following Karen.
Now this might sound retarded, but

And then for the movie, Karen is unambiguously the main character, to whom Hikari is playing the supporting role, while all the other girls tie up their own loose ends in the background, more or less.
I've grown to appreciate the B-gumis chattering and all the lead up to the second half of the movie much more on subsequent watches, too.