Do you think it's ok to influence eighth grade girls to dress up more mature?

I think there's a difference between the kind of "mature" outfit you described and "trashy." It's natural at 14 to want to look attractive and more adult-like (she's on her way to becoming an adult, after all!), and I don't see anything wrong with her grandmother helping her dress nicely. To me, that sounds like a fun day out with Grammy. :)