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As Maxpowers noted, there should be buildup on both outer pipes so if it is just one your NPP valve for that side may be stuck partially closed.
Direct injected gas engines create a lot of particulate matter and it will build up on the exhaust tips. The center tips likely build up more slowly unless you have your driver mode (or exhaust) primarily in touring mode. Even if both the center and outer pipes see relatively equal exhaust flow over time I suspect the center pipes will stay cleaner because more of the particulates will stay within the sound baffling matrix rather than making it to the exhaust tips. In any case the buildup is normal but it should be relatively equal side to side so I would get it to the dealer before you clean the tips. This isn't likely to be an engine issue so you are likely to get the "no code, no problem" from a less than stellar service department.
I am sure at some point a customer has gone in to a dealer with an obviously flat tire but if the TPMS system didn't indicate it was flat the dealer service department tried to send the customer on his way with the no code, no problem line.
