My 18 year old granddaughter doesn't care about her appearance?

she refuses to brush her hair everyday and always dresses grungy (i.e. she always likes to wear t shirts and pajama bottoms..and even goes in public like that. i am very particular about my apperance and i give her advice as to how she needs to present herself as she is beginning college in the fall. i worry because she seems not to care and has low self esteem. she is a beautiful girl and could even model if she wanted to. how can i guide her to try to improve her appearance?