When I met my boyfriend's parental unit for the first time it was sprung on me, like 'ooh I know come meet ma and pa, it'll be good'. Bad, distressing times. I was wearing extremely low-rise glittery trousers (they're ace I swear) and a crop top that was also quite booby. His dad was initially very confused. But on the whole I don't think anyone gave a flying foo.
Wear something COMFY, probably something you've worn before (nothing worse than wearing a new outfit at a vital moment and realising it doesn't fit right). Then concentrate on charming her with your wisdom and wit.