The joke is that localization teams often (especially in the past) dub dialog in English versions to replace Japanese dishes who's names, content, and social context English speakers may not know, with "westernized" food names. Onigiri is often called "jelly donut" as western children aren't generally as excited about a savory rice based snack or know what onigiri is. I've also seen yakisoba called "pizza" or "spaghetti," ramen as "soup" and in some dubbed versions they'll even animate over the original food with something else to match the dub.
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· 7 years ago
TL:DR the joke extends from Pokemon (English dub version) where Brock calls the obviously onigiri a jelly doughnut.
(Stuffed onigiri is available in all convenience stores in Japan)