The most I've ever considered wearing in public, though I've only done it when I've been attending a con, would be my Cardassian Union pin. I suspect it would be one of those things that only fans would recognize but others would think was just a neat, decorative piece of jewelery. I have actually had people who didn't know Trek, at the con (general sci-fi con) assume it was just "innocent" Art Deco piece. So that I might actually think about.
As for T-shirts and stuff like that for any franchise, I think people consider that merchandising rather than a costume. I may not like what you're advertising on your shirt, but it's perfectly normal to wear it (as long as there's not vulgar language on it, which I think is inconsiderate of young children if you're not in an 18+ venue).
But costumes...no. That would be as stupid as one of those sports fans that paints their face and chest doing that as everyday wear. Just wearing a sports jersey--just the jersey--that falls in the "merchandising" category.
As for T-shirts and stuff like that for any franchise, I think people consider that merchandising rather than a costume. I may not like what you're advertising on your shirt, but it's perfectly normal to wear it (as long as there's not vulgar language on it, which I think is inconsiderate of young children if you're not in an 18+ venue).
But costumes...no. That would be as stupid as one of those sports fans that paints their face and chest doing that as everyday wear. Just wearing a sports jersey--just the jersey--that falls in the "merchandising" category.