Anything one wears can be considered a "costume." I'm currently wearing my "not going anyplace" costume. In the closet I have a couple of "job hunting" costumes, and many "I'm being a responsible adult and working for a living" costumes. Then there are the "I'm doing yard/house work" costumes, etc.
in other words kostumo is better translated as outfit or getup than costume?
How is kostumo different from vestajxo(j)?
A kostumo is defined as a garment typical for a situation, purpose, office, ethnic group etc. So I guess outfit could be a good translation in some circumstances.
Vestaĵoj is more general. You would not use kostumo for underwear or stockings, and not for things that are just part of a kostumo.