Well, I will be wearing a kilt on Halloween this year but the only costume I will be wearing is "father who wears a kilt every day" while I escort my children around since I wear kilts exclusively when not at work. If I were to wear a costume and the costume itself did not dictate what to drape around my lower half then I would wear a kilt because that is my normal clothing. By strict definition, ALL clothing is a costume.

cos·tume (kŏs'tūm', -tyūm')
n.
1. A style of dress, including garments, accessories, and hairstyle, especially as characteristic of a particular country, period, or people.

2. An outfit or a disguise worn on Mardi Gras, Halloween, or similar occasions.

3. A set of clothes appropriate for a particular occasion or season.