I think what people mean is that she doesn't fit THEIR mental image of Lavender. I mean come on, you didn't seriously read the books with this image of a blank silhouette as Lavender just because J.K. Rowling didn't give any specific images, did you?
I get that people may not really have much reasoning to their complaints because the casting directors didn't have much to go on description-wise, but as a reader, I completely understand.
We bookworms have to complain. It's a whole world in our heads and it's difficult to see the "official" one look so different.

Lol actually your last name usual has bearing on what you look like due to language/ethnicity/race/culture/location/etc. For instance, you would envision someone with the first name "Maria" or last name "Gonzalez" to look Hispanic.
I get what you're saying though.
&MEGAHERTZ;

And most people are saying that she is suppose to be black. Why?
Because in the first movie, a black actress was credited as Lavender. Many people have learned that bit of trivia and thought that it meant Lavender was black in the book as well.

It's all like some bad movie.