DD2 8 very interested in looks and comparing herself with others

[wordassociationfootball]

'She's pretty isn't she?'
'Do you think x is pretty DD1?'

'Am I pretty mummy?'

I tell her she is very pretty/beautiful/ lovely looking and often follow it with and you're very thoughtful/generous/ often kind which is really more important.
But I feel uncomfortable with how often she asks. Also she looks in the mirror a lot in quite a posey way, sometimes when I'm washing her hair and sometimes when I'm reading to her. It gives me the pip actually. I've told her if we're reading, we're reading and that she can look in the mirror in her own time.

Yesterday she asked me if she was vain. I said I thought she was a bit. Said it wasn't a terrible thing and agreed with her that I am also a bit vain.

i expect it's born of insecurity, being a second girl, comparing herself. But DD1 doesn't seem half as interested in what she looks like.
I've been on the brink of shouting 'stop gawping at yourself in the bloody mirror' on occasion How can I encourage good all round self esteem? Would you accept this preening? Encourage it even? DD2 is very slim and athletic, a really lovely shape, do I maybe sound jealous Any good books?

G'wan..... talk to me

I don't have tons of advice OP, but maybe it's just a passing thing/awareness, and it may be better just to change the subject and not get too caught up in it yourself.
Maybe she is influenced by conversations among other girls at school? In a similar vein, my DD (9) worries me when she keeps admiring a girl at school because she is skinny, and wanting to be as thin as her. I don't know where she gets this from, as we have no TV or magazines (adverts) and I never diet and am a healthy 8/10. I think she just picks up on other children's concerns. I do worry about it, but I try to ignore it, otherwise I feel I am making it seem even more important.

I dunno, I think you are over thinking it maybe.
Would it bother you if she said
"I'm a fast runner! He's a fast runner. Do you think I'm a fast runner, mummy?" coz I get variants on that from my 5yo DS.

If he asked it all the time I would ask him why it was so important to him. Which is an opportunity to talk about what is and isn't important in our self-image, in what we are or aren't, and what matters in how we present to the world.

"Beauty is in the Eye of the Beholder" comes to mind.

yes, 'stop gawping at yourself' is reasonable.

check for any bullying at school, then if all well explain to her that how we look is trivial and that people who value it too much get rapidly very boring.

also make sure she's hearing the right message; no-one going on about diets, bad hair days etc etc.