trashy. I think these people feel that they can get away with it as it's making a statement - it's well known that they're rich enough to afford the upkeep so they're 'choosing' to be casual about their hair.
If I did it, it would look like I couldn't afford it, which would look trashy. I think it looks bad on anyone. Get a hair colour that isn't so obvious when it grows out if you don't have the time to visit the salon regularly