The answer to this question is most definitely 'yes'. The right hat makes you stand tall, improves posture and makes you feel fantastic.It will complete your outfit and you will ooze style and confidence.(Admittedly I do have a vested interest, in that I own a hat hire business), however when you consider how much time, effort, and most importantly, money the bride/groom and parents have undoubtedly splashed on their special 'once in a lifetime' day, the least their guests can do is to make an effort.
Obviously if the invite specifies 'no hats' or it is a very casual affair then you may wish to pass.

There’s a generational divide on this one. The mother of the bride will probably be wearing the kind of hat which precludes kissing (blessed relief, some might say), while the younger women will have their hair on the loose, perhaps decorated with a fascinator (one of those feather gadgets that flip about in the breeze). Grand weddings - ones with tail coats, top hats and helipads - tend to require the wearing of hats, particularly if it is happening in church. At one wedding I went to recently (in a field on the Isle of Wight), everyone wore wellies and false moustaches. So it’s horses for courses, really. Though, on the whole, I favour a hat. It lends a haughty insouciance to an outfit – and, hurray, is another excuse to stop at the shops.