Are they all dead at the end?
For most of the movie, anyone who gets splurged or hit with a pie, they're 'dead' (or out of the movie), so at the end once virtually everyone bar Bugsy and Blousey have been annihilated with cream.
I note the lyrics of the closing song are past tense: "We could have BEEN anything we WANTED to be," so wondered if this was them ascending happily to the post-pied realm?