Sadly, Mia showed poor manners by inviting some classmates in front of others. She may have a magnetic personality, but a poorly developed sense of etiquette. And your daughter has a normal sense of "fairness" or justice for her age group. This is one reason that teachers often prohibit handing out invitations unless they're given to the whole class (a source of occasional rants here on mamapedia).
The "looking stylish or pretty" thing is sad, but hard to avoid in the flood of materialistic and narcissistic messages given in television advertising. I actually managed to avoid much of that by restricting commercial TV viewing when my daughter was little, and getting rid of the TV when she was in 4th grade. Her needs and wants dropped significantly at that point.
Can you help your daughter plan her own party? Not as a form of competition, but as a way to help her find greater control over her own happiness. If you do this, be sure to either mail invitations, or hand them to other moms at school. And let your daughter know all the wonderful things that you notice about her, things that run deeper and count for more than outward prettiness.
You might find the following thought-provoking article about praise and motivation helpful: How NOT to Talk to Kids, by Po Bronson: http://nymag.com/news/features/27840/