In reaction, Frasier often rolls his eyes. Quite cleverly, the writers establish early on that Daphne--out of so many women in Frasier's life--is not a contender for his heart. She is, well, the eccentric English home worker who becomes a kind of sister figuratively before becoming his sister-by-marriage literally.
So Frasier rolling his eyes and getting sarcastic is par for the course.
But Niles with his temporarily unrequited love must appear less critical. If he rolled his eyes and got sarcastic, he would lose his status of "smitten."
Skilled David Hyde Pierce gives Niles the perfect reactions. He listens to Daphne's eccentric monologues with a kind of tilt to his head. He is listening and his brain is likely registering what she says as "odd" or "weird" but he is cataloging and contemplating the information, sliding it into his overall study of Daphne.
It is very adorable.
Of course, after their engagement, Niles, while still the somewhat perfect listener, is more willing to express surprise:
Niles: You don't mean the nasty, plague-propagating vermin, do you?
Daphne: No. I mean purebred rats, as in Siamese or Himalayan or Husky. My most prized one was an Andalusian Blue named Lady Prissy - and she wasn't sick a day in her life, so don't go blaming all rats because of a few bad apples!
Frasier: A few bad apples? Daphne, they spread a disease that nearly wiped out half the population of Europe!
Daphne: Shows what you know. Those were common European brown rats.
Frasier: Yes, but the point is-
Daphne: Oh, no, no, no! I'll sit here and listen to you prattle on about wine and opera. But when it comes to rats, you're in my house.
Niles: I'm not going to put my hand down there, there could be rats.
Frasier: Maybe they're just down-on-their-luck show rats!
Niles: I know, what was that?
Frasier: Don't ask me, you're marrying her.
