Lol, the worst "John" someone can give is "I'm tired" or "I'm just off". However, if they really mean it, then I can understand. It never does feel good to beat a guy when they are off their game or aren't 100%. In fact, Light, if I were you, I wouldn't even use that as much of a confidence booster. Say if this guy was true to his statement, came back around the next day and kicked your ***, then that would shake your confidence. Personally, when someone tells me that, I just keep in mind when I play him the next time. If I beat him again, awesome, if I lose in a close match, still great, if I get *****, then I guess he was telling the truth.
I can relate to being off my game. Doesn't happen to me as much in Brawl as it happens to me in Basketball. I definitely do get pissed when I can't get a single shot to go on a bad day.