Midriff-baring outfits are popular in real life, with outfits ranging from tube tops to two-piece swimsuits. Comics and cartoons seem to love this, since it's an easy form of form-service that the Censorship radar invariably misses. Within the last decade, the belly shirt seems to have become some sort of visual shorthand for "teenage girl". Or, in some cases, just "girl" period.
Note that simply showing the belly and a little skin around it, like a slightly open shirt, does not count. But a dress with a large section around the belly cut out does. Extra points if the bare midriff shows the infamous "v-shape" near the bottom of the torso.