If you have your own car it could take an hour or more to drive from DLR to SM. I always found traffic in SM to be bad (my doctor's office is in SM) not to mention the parking is bad around the pier. The most economical and best way would be to rent a car. If you do decide to visit SM you might want to check out 3rd Street Promenade which is really close to the pier and Venice Beach as well.