Does "or" means "otherwise"?
Here is the context, I hope it'll be helpful
Most car ads talk about the car or the life-style of its owner. VW's infectious spot does neither. Instead, it shows two slacker twentysomethings cruising aimlessly around a desolate stretch of Los Angeles in their VW Golf to the monotonous ditty Da Da Da. They retrieve an abandoned chair from the street, only to discard it minutes later after detecting its stench, then drive off, leaving viewers with this message about the car: "It fits your life or the complete lack thereof."
refers to "your life."
The meaning of "or lack thereof" is, "the absence of it".
For example, in the sentence, "You should use the system's technical description to identify the faulty area by its behavior or lack thereof", the meaning is, "... identify the faulty area by what it does or fails to do".
This expression is often used in a sarcastic tone.
In the example, "It fits your life or the complete lack thereof", the author could have said, "It fits your life or the complete absence of it". The underlying, sarcastic meaning is, "This car sucks, so does your life".