Was he insignificant?! Not by any means. Your Gpa didn't fly planes, or draft war plans, but his job was important. SK1 is the abbreviation for "Storage Keeper, 1st Class," meaning he was in charge of the supply and inventory of aircraft equipment and ordinance. On board the USS Tripoli, this was crucial; CVE-64 were light carriers sometimes dubbed by servicemen as "jeep carriers," because they carried small and unconventional aircraft (and sometimes, in the case of 7th fleet jeep carriers in the Pacific during the Battle of Leyte, extra equipment for landing operations).
The USS Tripoli was commissioned for the Atlantic, however. Your grandfather was charged with keeping track of and ordering the parts, ordinance, and equipment necessary for the operation of the seaplanes that were flying missions around the clock. These seaplanes were not long-range aircraft. They were short-range planes that flew reconnaissance for German U-boats, and were sometimes equipped with the weaponry to sink them (or else radio to a nearby battleship to drop charges).
Your grandfather's "certificate" is interesting. "King Neptune" was a name for a number of things relating to the crossing of a maritime boundary, traditionally the Equator. The origin of the ceremony is found in the British navy, but was adopted by the US navy where it subsequently became more fun. In the US navy, the ceremony was more like a party and was called "Shellback Initiation." Basically, a sailor who never crossed the Equator was considered a rookie, or "Pollywog," and they would have to do the veterans' dirty work. When the Line was crossed, a party was thrown for the rookies where the Pollywogs were subjected to a hearty hazing at the hands of the veterans, or "Shellbacks." The fattest sailor on the ship presided over the ceremony, and he was called "King Neptune."
Though reports exist of some violent instances of hazing in these rituals, most WWII veterans I've had the pleasure of talking to remember it fondly (I've conducted interviews with several of them). No doubt your grandfather does, too.