Link if this has been answered before ; I couldn't find it in my quick search.
Does it also depend on what stage you're at with her in the M3 model? I'd imagine the earlier she asks you, the more likely she's just seeking validation or trying to determine if you're a sucker she can use, or testing how comfortable you are with female beauty.
But in my recent case here's what happened : a mutual facebook friend commented on a pic of me with one of her friends on each side saying "ooh who's that hunky man? Introduce Haha." Waited a couple days, friended and she started messahing me, I took control and got her qualifying herself, then at some point she said "do u think I'm pretty? I think you're cute." I responded "I admire your courage. Look, beauty is common, u def have it, but I'd have to see how I like u in person, so far so good

."
That seems to have been a good enough answer for her, she's still engaged in the convo. But it got me thinking of how to respond to this sort of question in the future, and how the ideal answer might vary depending on your M3 stage.