It's a garment!
Okay, let's try to stay real, here.
I have had several guys say things like, "sorry Bear, I wore my kilt fully traditional but it was a wedding." Or, "Don't tell Bear but ..." and go on to say how he wore his kilt with a jacket and tie.
Just because my motto is, "kilts are garments, not costumes," doesn't mean you shouldn't wear them traditionally. Kilts are a traditional garment. If you get all dressed up with a plaid and ghillie brogues, I'm not going to be disappointed, nor should you care what I think one way or the other!
"Kilts are garments, not costumes," has more to do with being free to wear your kilt casually than restricting its use traditionally. There are some guys who wear their traditional wool kilts to our Kilts Night with untucked tee shirts and piper hose.
The look works. That's what matters.
Wearing your kilt as a costume has its place and there are dress rules for such occasions.
For casual wear, ...