I think the difficulty is that the game suggested ancient tomb and other clothes which are pure and cool, but pure and cool does not count. You need sexy and warm clothes instead of pure and cool. Just go for everything which is gorgeous and not simple, elegance and not lively, mature and not cute, sexy and not pure and maybe something warm :)
Comment found on Love Nikki-Dress UP Queen: princess 9-4
Not sure what items you have, and I'm assuming that you are using Momo's tips? The Mori Girl and Reindeer dress is a great item to wear. Also, focus on Lively, Pure, Simple, Cute, and Warm clothes.
Comment found on Love Nikki-Dress UP Queen: Im stuck at princess 4...