The Japanese were probably the most egregious overclaimers among fighter pilots. Mostly, I think, because they didn't really have an established method for claiming kills and, at least for most of the war, kills were given to the unit, not the individual.
The Germans were pretty good at times, but they still claimed three times as many as they got in the Battle of Britain. During the first month of the battle the British actually claimed less than they got, but they made up for that in the subsequent two months by claiming about two and a half times what they actually got.
I do thing, to a degree, there is a morale factor in letting your pilots and crews claims stand when you know they are bogus. Certain the USAAF's 8th air force knew that the kill claims by the bombers were wildly exaggerated, by about a factor of ten, but for morale purposes they let them stand. On the other end, the Luftwaffe under Hermann Goering took the Luftwaffe's claims in the Battle of Britain at face value and insisted the RAF was down to its last 50 Spitfires.