Around here you'll see a variety of dress at a "Halloween party" ranging from actual Halloween costumes to ordinary dress.

I've worn Highland dress to such things a few times over the years. I don't think it's necessarily perceived to be a "costume" but more on the order of "fancy dress" which also is seen. For sure it can be perceived as a costume by some.

When people are dressing "in costume" it's usually a recognizable character from some movie or what have you, rather than the traditional witch/ghost/vampire sort of thing.