Maybe, if it was a question about social status. Unfortunately it's a question about how you, when you're writing, describe your character's appearance.
For example, I could write that Angelique walked down Diagon Alley wearing a goth dress, or I could write that Angelique wore a black dress with a laced bodice and flounced skirt.
It's the same description, but one relies on a stereotype to convey the image. Basically, a character is more likely to be a mary-sue if she is trying to identify with a "cool" subculture, and this type of description prevents the person writing the character from having to actually research that subculture.
Even if it was about social status, a negative value would be dubious, since in the UK most schools are required to wear school uniform, so the cool and uncool kids can't be identified from something like the way they dress. There is a similar question about how your character treats people from other houses, though, since that's more pertinent to the wizarding world.
Just to let you all know I'm not perfect, here are my scores:
Selwyn - 25
Caelani - 34
Angelique - 52
Ethel - 27
Lucy - 32
I don't know how you're all getting such low ones.