Wooooah! Check out that zoom between panels 1 and 2! Who directed this, Mario Bava?
I like how this joke is not a simple reveal, but a puzzle we must figure out, including understanding where the characters are in figuring out the situation. Even if you have figured out that Ellen is allergic to the flowers (funny), you still have to figure out that Jon doesn't understand this, even though his own "bless you"s are our only clues (funnier), and that Garfield has pieced this together and is smarter than Jon (funniest).
Oh, and by the by, I guess Ellen is going to be like Mrs. Columbo, or Diane on Twin Peaks. The Ellen mystique will be that we don't get to meet her. She's a interesting character anyway, since she likes Jon enough to speak on the phone every day for weeks, but not enough to go out with him.