Still doing some diving on nakshatras so I can make sure I know what I'm talking about before I do my serious Genshin astrology posting, but in the interest of getting more active here, just some potential placements in my mind:
Yanfei might be an Uttara Phalguni or have strong placements there, which tracks for her July 24th birthday.
Kuki Shinobu has to have some sidereal Virgo going on, I'm leaning hard toward Chitra, possibly Ascendant.
I also think Kaveh and Keqing likely have Chitra somewhere, Kaveh more the Libra side (architecture, design), Keqing the Virgo side (she does city planning and is often likened to a tigress, the yoni animal)
Venti/Barbatos is textbook Swati, and the Swati themes echo through Mondstadt itself (winds/breath of life, scattering as dandelion fluff, the strong tradition of music)
Diluc has some intense Aries influence going on, even though his Sun is (in a 0 ayanamsa) in the Taurus side of Krittika, his aesthetics & being Teyvat's equivalent of Batman suggests some Bharani and possibly Ashwini as well.
Tartaglia is the most blatantly Purva Ashadha character I think I've ever seen, but I haven't gotten to digging deeper on PA yet.
Baizhu has either Ashwini, Shatabhisha, or both going on, but one way or another it's hard to ignore the heavy Shiva symbolism in his design & characterization.
Hu Tao may be a Revati Moon. Haven't gotten to that nakshatra yet so it's on the backburner.
Not sure yet what Faruzan's placements might be but she gives strong Mercury nakshatra energy.
Wanderer has been a tough nut to crack, he gives Jyeshtha & Ardra vibes.
Arataki Itto may be a Magha Moon or Rising.