I can understand the frustration-- as a "jeans 'n t-shirt" girl myself, getting dressed up is... well, a frightening concept (makeup? eep!)
But I also see the need for at least one rush event to be quite dressy-- first impressions do make or break a PNM's rush experience, I think.
I think voicing concerns about how the event will project to PNM's is a valid one, but definitely do it in a diplomatic, kind, and concerned fashion.
For what it's worth, whenever we had one of our dressier events, somehow a few of my sisters and I actually wound up having a "let's get Mel (being me) dressed" sort of party beforehand... which I griped about at times, but wound up being hysterical fun.
Perhaps it'll ease your worries about it to propose some sort of similar pre-event gathering of sisters who are willing to help less fashion-sensed sisters in a "dress for success" sort of gathering.
~ Mel.