I agree with Jock and Ron.
Nobody will criticise you for wearing the kilt, though be prepared to be taken for an American tourist. There is a saying here in Scotalnd that a kilt is a garment which Scots only wear while visiting North America and North Americans only wear while visiting Scotland. As a Scot who often wears a kilt here in Scotland I have been asked more than a few times if I am an American.

There are very few people on Barra and a clan gathering will obviously be big news on the island and I would envisage the locals will be expecting the diaspora to be kilted up as is commonplace at clan gatherings.

The very fact that you are in the audience at an Edinburgh Military Tattoo marks you out as a tourist. As a Scot living just 55 miles from Edinburgh, I personally would never wish to waste my own precious time and money attending the tattoo, but I appreciate that the tourists love it and it brings in a lot of trade into Scotland. Wear what you like, you will certainly be far from alone, there will be plenty of other kilted American tourists going to the tattoo.