I go to church in jeans every week. Some of the older congregation are in jeans too, some are more formally dressed. My dad always wears a suit and my mum always wears a proper dress but no hats.
The royals do wear hats on all kinds of occasions when nobody else would though.
I have no issue with her being called Kate Middleton either. What is her surname if it isn't Middleton? Might be Cambridge, might be Wales, might even be Windsor but AFAIK officially she didn't have a surname to change to on marriage as all of those are changeable. And if Henry is always known as Harry she can continue to be known as Kate.