The interesting thing is that Elly ignores his attempts to try to pull his foot out of his mouth and say that she's the one pretty lady that he managed to work up the nerve to talk to. A normal woman would let John do something like that because she wants to look in the mirror and like what she sees. Elly, despite what she might believe, is anything but a normal woman. It's fairly obvious that she needs to think that John married her not because he found her attractive but because he needed a housekeeper. If she started thinking that he married her because he loved her and thought that she was the most beautiful woman alive, she couldn't feel as if her life was an endless tapestry of despair and humiliation and that would be just terrible.
The reason for all of this is that Lynn is a frightfully insecure woman who needs to have it confirmed that she is indeed fat and ugly so she can have something to feel awful about. If she were to be convinced that she was reasonably attractive and always was, she'd have to admit that she made herself miserable for nothing and that would be all kinds of wrong. Better to think that Rod had an agenda than to feel good about herself.