Well, Sculpt, it seems that Carrie's constant imbalanced relationship with her mother creates in her an extra-sensory sensitivity towards social expectations, and that can affect someone's self-image (even if she doesn't specifically think she's ugly in some way).
Here are a few scenes from the 1976 Carrie:
1. Breaking the mirror in some kind of act of mental self-destruction
2. Attacking her mother with flying knives
3. Her social embarrassment in the opening girls high school shower-tragedy scene
Here's a quick short-story about an 'ugliness psychology' perspective on the Carrie/Leatherface duality:
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Carrie stood soaked in the blood of many US Navy men who had raped her and across her in the hallway of the Navy base was the chainsaw-wielding maniac Leatherface.
CARRIE: I just killed a dozen Navy-men!!
LEATHERFACE: Eat this!
CARRIE: You're sooo ugly!
LEATHERFACE: Uh-uh!!!
CARRIE: Need a mirror?
LEATHERFACE: Woo!
CARRIE: You're dirty!!
LEATHERFACE: Food!
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