One tuna mayo onigiri is 197 calories, which is about the same as a slice and a half of bread. The rice is 90g and makes up 140 of the calories. The tuna mayo filling is only 15g, about 45 calories, and the nori and seasoning are the last 10. It is a small, dense carb snack with almost no protein, 4g, because the tuna is mostly mayo by the time it goes in.
The 'mayo' part gets blamed, but the difference between tuna mayo and grilled salmon onigiri at 7-Eleven is 23 calories. The filling is too small to matter much. What matters is how many you eat. A konbini lunch of 2 onigiri, a karaage-kun and a sweet milk tea is about 750 calories with 20g of protein, which is a poor deal. Two onigiri and a salad chicken breast instead is 520 with 30g of protein for the same 700 yen.
Onigiri are a decent snack in a country where the alternatives are 400 calorie melon pan and 350 calorie sandwiches. The trouble is the rice is refined, the protein is low, and hunger comes back in 90 minutes. Pair one with a boiled egg (70) or a stick of cheese (60) and the 260 calorie combination holds you much longer than a second onigiri at 394.