I and many of my friends have worn gothic and lolita clothes in Japan. Being Japanese or foreign doesn't seem to matter. The two most important things I've found are...
1) How you look in the clothes.
Your race is a tiny thing in comparison to whether you look good in what you're wearing. If you wear the clothes well, you'll receive many compliments like, "Oh, how pretty! You look like a doll!" But if you look bad -- poorly fitting and mismatched clothes, bad posture, sloppy hair and makeup -- yes, people will make fun of you. If you're not sure whether you're pulling off the look well enough, submit a picture of yourself wearing one of your outfits to the most critical English-language lolita internet forum you can find. If you pass muster with the online communities, you shouldn't have any trouble in Japan.
2) Where you're wearing the clothes.
Regardless of how popular they might seem from overseas, gothic and lolita clothes are weird in Japan. You can wear weird clothes in the parts of major cities where other young people wearing weird clothes are hanging out. If you wear lolita in the suburbs or countryside, you'll stand out and people will think you're strange. This is true regardless of your race.
In short, I wouldn't worry about being a foreigner when it comes to wearing lolita in Japan. Just make sure you look your best.