Is onigiri a good party dish?

It all depends on your guests. If you have friends who are knowledgeable of Japanese food, or who are just adventurous eaters, its a great idea, but if you have a lot of picky eaters you may end up with a plate of Onigiri to finish alone. A happy medium might be to fill the onigiri with something pleasant to picky eaters and onigiri lovers like. Maybe chicken as opposed to pickled plum.