Can the mother of the groom wear pants?

It is up to her. If you're speaking about American 'pants' (English Trousers) then it is her decision as to whether she wears pants, a skirt or a long dress. If you're speaking about English 'pants' (as in underwear), then most of the time she should - as it is just common courtesy - especially if it is windy. No one wants to see their Mother's bits.