There's always one WBD snob...
It's a day about books and reading not costumes, it doesn't matter what the children wear or whether someone on the internet thinks their costume is acceptable based on some arbitrary opinion of what is/isn't "worthy".
Some parents cannot afford a new costume or props so they use whatever costume they have.
Some parents don't see the point of buying a new costume or pointless props, I know a few who haven't for environmental reasons, so again they use whatever costume they already have.
Some children don't like dressing up days or will dress up but only in certain costumes. DS1 school has declared it "Harry Potter Day" this year and only HP costumes are allowed, DS has gone in his normal clothes as a Muggle because 1, we don't have any HP costumes and 2, he hates dressing up. DS2 has gone to his school in a Muggle costume too because he also hates dressing up.
Some parents don't have the time to sort out costumes so use whatever costume is already in the house.
Some parents have chaotic lives and don't give a shit that it's WBD so the child wears whatever costume they already have in the house.
And, finally, some children like dressing up as Transformers and why should they be told no when it's a dressing up day?