If a costume suddenly impairs a cat’s vision, they’ll get scared and start panicking because they know they won’t be as efficient at defending themselves.
If the shirt, sweater, or Halloween costume is made of poor materials, it’s too heavy or tight; then your cat can feel uncomfortable and sometimes experience pain.
Some cats might have a more unconventional reaction and freeze after getting dressed up instead of fighting or fleeing. This could be their peaceful way of telling you they’re not enjoying this situation.