I'm of the "If it is made to be a kilt then it is" camp. MUG might apply as a general category, kinda like pants can include jeans, capris, slacks, and many other variations. Of course so will the term skirt but the connotation of skirt is feminine.

I don't mind the term MUG as a term, but if it looks like a kilt, if it is made like a kilt, if it is supposed to be a kilt then it is a kilt.

My problem is not when someone calls it a skirt because they don't know any better, but when they call it a skirt but know it is a kilt. It took my dad a while to stop calling my stillwater a skirt and he still does sometimes. My mom calls it a skirt sometimes but she has just been silently supportive even though she doesn't like the thing. My grandpa, who I had talked to about this when I was talking to my grandma (he wife) about her possibly making a kilt, calls it a skirt is an almost mocking tone.