We always bring pirate costumes and Halloween costumes. We enjoy it.
There will be alot of families dressed up, adults included.
If its not your thing, you def won't be the only ones not dressed up.
As a PP mentioned some people go ALL out with their costumes, some family themed costumes (one year we are all Star Wars characters and the other family traveling with us was Wizard of Oz), some extremely elaborate hand made gowns etc) and others have simple costumes.
Do what you feel comfortable doing and you will be fine.