That's tons of fun! 
My first few attempts were comically bad, but then I got the hang of avoiding the booms.
Maybe you could add something like a maximum point value per level, such that the level ends once enough moles have been presented with that point value. (Like 10,000 possible points will be presented). This way users can get a % completeness score. Also with levels you can gradually increase difficulty like time the moles are presented, amount of booms shown, total points to get, etc.