So I think Kirishima's teeth are sharp because when he was  younger and nervous he had the habit of biting his nails. Obviously he's not weak so his bite is strong, causing his nails to break/his fingers to bleed.

I think he would use his quirk on his fingertips so it wouldn't hurt. When they were hard he would chew, over time his teeth were filed down by his harden fingertips. (why does that sound so wrong) Thus becoming sharp.

Thanks for reading this random theory thingy. I don't know what I'm doing anyway thanks and tell me what you think about his teeth. (lol that sounds really weird)