Anything goes, as long as your bike doesn't look like it's the most expensive bike in the rack.

Some people, particularly teenagers and students, can be quite creative in dirtying up their bike. A friend in school had glued sand to her bike, and then painted it. Quite a unique look. But it got stolen anyway, and when she found it, the thief had scraped all the sand off.

But there are many reasons why a bike might look ratty. The owner really could have had that bike for a long time, it could be second-hand (or more likely fifth-hand or something), it could have been stolen, it could have been found in the trash and refurbished, it could have been ratty'd up to prevent it from getting stolen, or it might be someone's artistic expression.

But in big cities it's certainly not unusual that the lock is the most expensive part of a bike.