Was it that, or were his adversaries making fun (dark fun) of the record he did have. If it is true that he had 20 combat missions in recce aircraft, that is nothing to spit at. The motto of the recce guys, at least later, was "Unarmed and Unafraid". (That's also the title of book about airborne reccon). While the first part was often true, the latter part was a damned lie in most cases. :)
A quick search for the story behind the "tailgunner Joe", nickname did not produce much results. The only explanation I could find comes from The New American, which if one is wont to be "down" on TJ, is probably not going to be very convincing. However here is an excerpt from the article there:
Although his judgeship exempted him from military service, McCarthy enlisted in the Marines and was sworn in as a first lieutenant in August 1942. He served as an intelligence officer for a bomber squadron stationed in the Solomon Islands, and also risked his life by volunteering to fly in the tail-gunner's seat on many combat missions. Those who quibble about the number of combat missions he flew miss the point - he didn't have to fly any.
The enemies of McCarthy have seized on his good-natured remark about shooting down coconut trees from his tail-gunner's spot (an ABC television movie about McCarthy in the late 1970s was entitled Tail Gunner Joe) to belittle his military accomplishments, but the official record gives the true picture. Not only were McCarthy's achievements during 30 months of active duty unanimously praised by his commanding officers, but Admiral Chester Nimitz, commander in chief of the Pacific Fleet, issued the following citation regarding the service of Captain McCarthy:
For meritorious and efficient performance of duty as an observer and rear gunner of a dive bomber attached to a Marine scout bombing squadron operating in the Solomon Islands area from September 1 to December 31, 1943. He participated in a large number of combat missions, and in addition to his regular duties, acted as aerial photographer. He obtained excellent photographs of enemy gun positions, despite intense anti-aircraft fire, thereby gaining valuable information which contributed materially to the success of subsequent strikes in the area. Although suffering from a severe leg injury, he refused to be hospitalized and continued to carry out his duties as Intelligence Officer in a highly efficient manner. His courageous devotion to duty was in keeping with the highest traditions of the naval service. ---- Well, at least I learned something I didn't know before. MaCarthy was Jarhead AND an Intelligence officer. No wonder he was kinda "different". We all know how Marines are and all the intell guys I know, (A bunch from the USAFR) are "different". Including the one "guy" who is a gal, and also one star general, the highest ranking USAF Reserve component intell officer, AFAIK, although she's probably the most "regular" person of the bunch. But none as "different" as the engineers serving in technical intell billets, we were really strange.