I would honestly tell her that if she dressed oddly that people will be looking at her and talking about her.
She needs to know how society does this. It is especially true at weddings. I mean, how many of us have never gone to a wedding and say someone wearing something odd and we talked about it all through the reception...just sayin, she needs to know that people will look at her and talk. Sometimes when we accept an invitation to attend an affair like this we do so knowing we'll have to dress up.
Otherwise give her the option of staying home or at the hotel and not dress up at all.
Her wardrobe sounds like an old woman's clothes but it may just be the words you used to describe it. At 13 she may need to go shopping and update it.
I don't know anyone who has worn coulottes for at least 5 years. Just consider trying to look at some magazines and let her pick some more fashionable clothing. It doesn't have to be expensive either, even Walmart has some cute teen stuff. But places at most malls have clothes that would fit a young lady and not be too mature.