Jon may not know what a dress code is (seriously?), but it probably won't matter, since he customarily wears a jacket and tie on dates anyway. There must be some note of social idiocy in Jon's voice that indicates to maitre d's that he is unfamiliar with basic rules of dining etiquette. Even better, it turns out to be true. Somewhere deep inside, I know this joke pushes Jon's social retardation a little too far: he not only doesn't know what a dress code is, but doesn't know what kind of clothes are for girls and which are for boys (seriously?).
Hey Garfield, so far as I can tell, Jon's had dates three nights in a row. And he may not know that ladies don't wear ties, but when was the last time you "got out"? And yes, Jon has a look of wonderment because a restaurant requires jackets, but still, good job sassing a guy who has a date by confirming that he needs a date. Witness the desperate lengths to which a lifelong curmudgeon must go when faced with a man making self-improvements.