Gretchen and Karen are just as 'weak minded' and 'timid' as the unpopular kids.
Gretchen and Karen (especially Gretchen) don't have the guts or courage to make their own choices or go against Regina.
Notice she doesn't EXACTLY what Regina says like a servant out of desperation just to be liked.
That's a huge sign of being weak and afraid. Despite being a member of the plastics.