Very nice idea, very well done
But I'm sure you've heard this plenty of times before. The ball moves too fast. It's very hard to hit exactly where you want to. I'm also sure people have asked you to add powerups, but never bothered to state possibilities. Not to criticize, but I'll try and suggest a few to the best of my ability:
Slow motion (easier to aim, maybe it'tll be temporary, or regenerates slowly)
Aimer (a line that shows where the ball is so you can watch the top of the screen, and shoot based on more than an estimate)
Sticky ball (Either it makes enemies slow down, or it leaves a slight line of gooze that acts like a moat, that lasts for a few seconds)
Anyway, those are the few powerups I could think of (suggestions, no offence taken if you don't use them or anything like that). Improvements would include more of a goal, or a sub-goal. What i mean is that since in bowling, the floor is ever so slightly sloped, and you usually aim for the center, the ball could follow the slight sloped rule, or more enemies could appear in the center, giving a slightly more bowling mindset.
The bowling ball also takes quite a bit of time to 'reload'. I read in one of your replies that the speed+firepower upgrade helps that, but I figured I'd mention it anyways. An addition to the game could be a sort of a charging thing. Instead of clicking, you can click and hold to 'charge' your ball, which makes it spin more, or veer towards the center, or just destroy monsters as a whole. It could let off a sort of sonicboom, freezing nearby enemies (near to the pins) for a few seconds, enough time to recover and fire at them.
I love the idea though, very nice game, looking forward to any future games from you, be they a sequel or another very innovative idea.