When John and Lorraine first call Mr. Pignati, he seems excited at the simple notion that Pignati is gullible and could easily be taken for a ride. So he does not necessarily like Mr. Pignati himself, but only the possibilities he presents.
When they visit Mr. Pignati to collect the $10 he had promised to donate to their made-up charity, John loves the big goofy smile Mr. Pignati is wearing. He relates the "crazy" smile to a drunken Santa Claus. He and Lorraine both think he is "too nice to be for real" and although later he considers that Mr. Pignati might be a psychotic killer, that smile on his face is what immediately makes John like him.