A first date is supposed to show that partners care about each other - that's enough.
A first date should be about getting to know each other more.
Though there is the dichotomy between trying to establish a lasting relationship (So wanting to get to know each other more deeply) and simply being interested in sex (So the date is just a pretense for "Do we like each other enough to consider sex?")
Saying "you are beautiful" is not the same as saying "you are 5 feet tall". It means "I'm telling you that you're special to me. I'm ready to open my heart and soul to let you in". If her reply to that is "I know" she's either a total moron or just indifferent to the person
Saying "You are beautiful" is not even close to saying "You're special to me"
It is a barebones compliment. It is not deep, it is not meaningful. It's simply a compliment. One that doesn't necessarily indicate anything special (Or even anything related to a romance at all... I've called plenty of my female friends beautiful when they've dressed up nice, simply as a friendly compliment)
Most importantly, it's not unique. Which is where the joke aspect comes from. It's such a basic complement that she would have heard it TONS (Heck, one of Wyll's banter dialogues is him calling Shadowheart beautiful... Before he gets shot down by both Shadowheart AND Lae'zel because of how lame it is as a way of wooing someone).
Her saying "I know" is a direct jab at you making such a unoriginal compliment.
Still, she's not the total Bitchy McBitchface you portray her as because she doesn't JUST say "I know" and move on, she immediately follows up with a much more sincere tone "But thank you for noticing" which is her acknowledging that you made a complement and showing appreciation that you did.
Sure she doesn't reciprocate, but not all compliments need to be reciprocated (And as I mentioned in a prior comment, she's still unsure if you're interested in a proper relationship that she's looking for until the following day, so she'd naturally err on the side of holding back) and instead she does the subtle invite to a kiss... Which means a whole lot more than a generic and meaningless "You too"
