Ooh this makes me feel better. On my first trip to Japan, I went through so much trouble and worry to cover my tattoos (I'm not even heavily covered in the usually-exposed areas, but my fiance is), and constantly wore long sleeves and even resorted to bandages. I'm going again at the end of September and am worried it may be hot. I know not to expect to go to most hot springs, temples, & water parks with them exposed but was worried about Tokyo Disney, where I will be visiting for the first time. You hear so many "horror stories" on the interest it is nice to hear from someone who has experience saying it's generally no big deal.
Regardless for the poster, I know Japanese women are more comfortable showing legs than ... chest area, which US has more of an infatuation with. I would be wary of visible bra straps and super short crop tops but that's about all. Light flowing fabric, both in color and texture, tends to keep you more cooler than clothing that leaves much of the skin exposed regardless.