This was by far the funniest gag in the whole movie. I loved the rest of the movie but this is so real that it hits us hard with this kind of thinking when your crush you wanted in your life is taken. There's no other dream gag so it had me fucking dying of laughter all these years later.
It's like if Walter Mitty or Doug Funny's fantasies were even more psychotic than usual. I'm not going to lie, if I ever wrote a TV show or movie about a hapless character, I would have used that fakeout fantasy trope to death.
as well this is a daydream not a real scene and done for comedic effect, most of the stuff in kick ass is just a gimmick for most shock value, to make the movie look better then it actually is.