If you ask and she agrees, it appears to create an implicit favor she was probably uncomfortable with.

I guess that would depend, among other things, on what tone of voice you use and on which ways you've interacted with her so far. (I've had people who I'm sure beyond reasonable doubt they're not sexually attracted to me offer to share their umbrella as recently as this week.)

Anyway, I remember being offered to share an umbrella by a complete stranger of the gender other than mine when I was in my teens (and I understood pretty much f***-all about gender dynamics) -- I declined mostly because I was in a hurry and didn't want her to have to walk as fast as me.