Annoying? Eveyone is entitled to "meet the characters"! Children do not have a exclusive entitlement to the characters. If you do not like the way it is set up, do not go on a Disney cruise. You probably complain when adults pose for pictures with the characters at the Parks as well. Disney is for children OF ALL AGES, not just little kids, as Walt Disney himself said. Without adults to pay for the trip, there would be no going on the cruise!. We pay a lot to go on these cruises that we love and we are going to try to get the full experience out of it as much as you are.
Just be patient and wait your turn. I am 60 ys old and get in line to get my picture taken with characters all the time and I have to wait my turn too. It is fun and enhances my vacation and I don't give a hoot if you are annoyed. If you let the fact that adults are having as much fun as your kids on the ship, ruin your trip, you are in for a terrible cruise vacation with Disney because adults have their own areas and restaurants and can pose for pics with characters as often and as much as they want. There was no law against it the last time I went on a Disney cruise (in April 2013).
Get over it and just enjoy the fact that you even can afford to take a Disney cruise. Many people can't.