From what I can determine, Makopi seems to be short for Makoto. Kind of like Liz is short for Elizabeth, or Bob is short for Robert. I'm not sure about the onnanoko no hi part though. Girl's day (also called hina matsuri) is a holiday in Japan, but I'm not sure what Girl's Day's Makopi means.
Ok, that makes a bit of sense. I hadn't actually noticed this girl's name was also Makoto (only Kannon's Makoto was coming up regularly in a search for まこぴー). It still doesn't make much sense though.
