As far as I know, she did fight Utsuho (who is pretty much an embodiment of the Sun) and won.
Cirno seems a bit of a dark horse in Touhou. She's come a long way from where she started as just a stage 2 boss. I still remember when she got her own spinoff in Fairy Wars. Man, that danmaku freezing was awesome! At any rate, I think she's a lot stronger than most people give her credit for. And it feels like she gets stronger with every go at it she takes.