The Blapblaps are a species who are extremely dangerous to Humanity...
... because of their high reproductive rates allows them to demographically steamroll every other species.
Yes, they're weak, yes, they look pathetic, yes, they probably aren't even all that smart, but there are stronger, competent, and smarter ones among them. Their threat comes from sheer numbers, and in war, that advantage is a game changer. It doesn't matter if you can absolutely slaughter an enemy in a war, killing hundreds of thousands of them with each battle, winning ten battles in a row, but you only have one army, they only need to win once, and other races would feel the attrition even if they won. It's just that the Psycho Warriors make inflicting this attrition unusually painful for them, but as was proven with Captain's Crusade video, and Xavery, that Messenger who "I don't have long! I come bearing an important message!" died on the way to getting there, the Blapblaps do pose some level of threat to the Psycho Warriors.
I don't think that Genobomb was wasted on those 20 billion Blapblaps at all, even if it was used inappropriately.