May be a bug with the NPC itself. Though I did that quest yesterday, she had been MIA beforehand. I actually came here yesterday to see if anyone else had the problem and came across this post.
I had spent several game days and a couple real days in Solitude, doing quests, etc. About halfway through my visit she disappeared and I was desperate because I needed to buy new spells and for RP reasons didn't want to just jet off to a different town. Plus it was bothering me that I couldn't find her. Along the way, one of the quests I took was the Sheogorath one. But as I said, she'd been missing before that.
Spoiler Of course she has a "little secret" that might keep her from staying up during the day, but it's odd since she's in court with the jarl no matter when you first meet her, AFAIK.