Ha, I don't even own a sewing machine! My dd is 18 now so dressing up days are well in the past for me- but when we did book day she was always at least as a character from a book- nothing elaborate, just cobbled together from things we had- Pippy long stocking, Goth Girl, The White rabbit, Winnie The Witch, Mog, etc.- all super easy with a few basic clothing items and a little face paint.
I work in a primary school- it's much nicer when they dress up and bring the book they are dressed as to school- children have time in class sharing the books they bring with each other which might introduce others to new books, discussing their characters etc. Not a lot to be gained from dressing in a football strip on book day except signalling to everyone your lack of engagement with the event.
Book day at my school is about trying to foster the appreciation of books and open up children to new inspirations for reading- we have visiting authors, a book swap, visit to the library, paired reading with an older reading buddy, a fancy dress parade and an amazon voucher for the most original costumes followed by a bookshop at the end of the day where they can use their book day voucher.
Lots of effort put in by the staff who dress up too and do a 'masked reader completion' for the children as well as reading their favourite stories to the children.
What a shame some parents are so negative!