The school I work at (and where my son goes) had world book Day, but people didn't have to come in costume. You could choose to just wear casual clothes. I let my 3 year old choose his favourite book, which was about a dog, so put him in brown and white clothes and painted a dog nose on his face with my eyeliner. And stuck 2 bits of felt to a hairband. Didn't cost me a cent and he was so happy.
Honestly I like the "no hard and fast rules" book Day.
Honestly as a person who works at a school, world book Day is by far my favourite costume day. Even if it was just a bit of face paint, the kids really got into character and were so eager to talk about their favourite books. Maybe we can look at this from the kids perspective. It's about imagination not an elaborate costume. It's about encouraging people to read actual books, not just a competition on who can make other people laugh on Instagram.