We are a subculture, not a costume.
It is not acceptable to dress up as a terrorist Muslim, it is not acceptable to dress up as a 'Squaw' (that word, by the way, is an insult to Native North American women) and it is not acceptable to dress up as a Chinese person with taped eyes and a coolie hat because these are all reducing groups of people to flat and sexualised or negative stereotypes and perpetuating those stereotypes.
It's not acceptable to dress up in a skimpy 'Goth' costume with a bad wig, white face-paint and cheap plastic 'collar'.
'Goth' costumes often present a sexualised (sexy-fied) stereotype of Goth, and one that errs on the 'Goths are people who think they are vampires' and 'Goths are slutty and into kinky sex' stereotypes a bit too much for comfort. There are also those who, like those who wear the racial stereotype costumes, wear them to mock those stereotypes, people who wear Goth costumes in a way that deliberately mock what they think Goths are. THAT really annoys me.
I live the Goth subculture 24 hours a day; even my dreams run to the same aesthetic as everything else I do, even my work clothes err on the Gothic. I have Joy Division and Dead Can Dance on my headphones when I sit on the bus. I have spider-web loo-roll and black plates. To say I am passionate about my subculture is a bit of an understatement, and to know people use Hallowe'en as an excuse to mock us is... well irritating to say the least. Especially when Hallowe'en is often quite dear to Goths.
Now, wearing a Goth outfit at Halloween isn't inherently inappropriate. I've actually helped a friend put together a proper Goth outfit at Halloween, but it consisted of clothes that would actually be considered Goth (from her wardrobe and mine) rather than a tacky and cheaply-made, store-bought costume, and she wore Goth makeup rather than white and black face-paint. Her intention was not to mock Goths, her outfit was based on what is authentically Goth in consultation with a Goth, and she wore her outfit as an opportunity to explore a different way of looking for a night. Yes, the idea that it can still be considered a 'costume' could still be grating to some, as to her it was something different to wear for a night and not a form of expressing her permanent inner self and subcultural affiliation, but I don't think that this was not done in any malevolence and what she wore was actually Goth and not a tawdry parody of Goth.
I know we are not actually 'scary', but a lot of Halloween costumes these days are not based around spooks and ghouls - yes vampires, werewolves, witches and zombies are always popular, but I see people dressed up as nurses, ladybugs, bananas, super-heroes, etc. A costume that is trying portray Goths as a scary entity alongside the zombies, vampires, serial killers and monsters is not appropriate and perpetuates negative stereotypes, whereas a costume that portrays Goth as just a different look is not a problem. Goths often find inspiration in the stories of ghosts, vampires, werewolves and other supernatural horrors, but it does not mean we believe ourselves to be like them, or that we ourselves are dangerous. Our aesthetic often draws from the artwork and cinema surrounding such mythology, and our tendency to seek beauty in dark places does mean many of us particularly enjoy Hallowe'en as a festival of spooks, death, monsters, etc.
Some of us may celebrate Samhain on the same day, a Pagan holy day, not because it is the Goth thing to do, but because there is significant overlap between the Pagan and Goth communities.
Note for non-Goths: It is not funny, cute, or amusing to ask, and even ruder to shout, "It's not Hallowe'en yet!" at a Goth, or "Hallowe'en was last week/month" at them. We know, this is who we are every day, and you are mocking us. Even the politest Goths get tetchy when they field this supposed 'joke' endlessly.You look like a rude and unimaginative idiot, and not even the slightest bit funny when you crack these 'jokes'.
I am aware that a lot of this is literally cut and paste from an earlier post I made on subcultural appropriation back in March, but I felt that this point would need repeating nearer Halloween itself, when those searching 'Goth Halloween Costume' may come across it. I did elaborate on it, too, so for those who read the previous version of this post, there are new bits to stop you getting too bored!