Muggings are extremely rare.
Please beware when you are walking on Sukhumvit at night, around the pedestrian overpass close to the Landmark. This area is home to a gang of Katoey who are notorious for pickpocketting. A lot of expats and tourists, however, never carry their passport with them for fear of loosing it. Please read Khun Bill's thoughts on carrying your passport or not . It might save you a lot of problems.