There doesn't seem to be any sort of story mode, so boss battles wouldn't make any sense.
I was thinking that boss encounters could be triggered by the same events that trigger the powerup unlocks. So whenever you get to the point when you unlock the pinball powerup, for instance, you play a boss fight that sort of trains you to use it, and then once you beat that fight the item is unlocked for regular play.
There were boss battles in LRB.
Not sure how that's related, but BOOM
The nice/mean guy.