I am not too sure what the history of jeans,tuxedo or jellaba are,but are you suggesting that they are any more than variations of "clothes"?
Nope. Neither is a kilt.

You are not suggesting that they are costumes,are you?
They aren't "only" costumes, but they can be used as such.

The kilt is also a piece of clothing,no more no less
And as such, they are as suitable as part of a costume as jeans, a tuxedo or a djellaba.

but a costume?No way.
Why not?

The kilt is an evey day choice of clothing for a Scotsman,should he choose
And jeans are an everyday choice of clothing for an American, and the Djellaba is an everyday choice of clothing for an Egyptian. That doesn't mean they cannot be used as part of a costume.

to suggest that The kilt is a costume shows a singular lack of knowledge of Highland attire and its meaning to any Scot wherever they may be in this world.
I don't think any said that the kilt is "just" a costume. On the other hand, to say that is cannot be used as part of a costume, IMO, moves the kilt from "everyday" clothing to ceremonial garb.

I do not wish to be rude,but an awful lot of suffering and more than a little blood has been spilt over too many years for the kilt to be regarded as a costume.
The same could be said for every military uniform on earth, and they get worn as costumes.

For those of us who have adopted the kilt as our daily wear, I think that it comes down to the fact that most attendees at a costume party pretty much desire to look stupid or funny! Most pick their get-ups for shock value. We, therefore, resent it when somebody chooses to do this with our beloved kilt!
I agree with that, with one caveat. I can see the difference between the guy who buys a 4.99 "Scottish" costume that looks like crap, and the guy who shows up to a costume party dressed up as a bagpiper, in a real kilt, etc, and looks good.