it depends what class you are doing - if it is best turned out then yes they probably would. In a showing class they probably would not get that close to the horse to check it out throuoghly. But don't be tempted to do anything about them, my old horse came to me as a 4 year old with aural plaques and my old instructor took a scrubbing brush to them when I wasnt around - the result, bleeding, infected ears and a headshy horse. His vanished after a couple of years. Headly Britannica has serious Aural Plaque and it never stopped her being a winner!
We have shown horses successfully at HOYS with aural plaque - we do try to colour them if we can in as the horses have their ears clipped out and if they have dark ears and white plaque it can show. But we have never been put down in a class because of them. I suppose in a best turned out class or something like that it may make a difference.