To me is seems Lazy, skin a bunch of existing characters onto a fighting platform, tweak the moves a bit and BAM you can prey on the nostalgia of an entire generation. I personally have never wondered who would win if Mario and Princess Peach would go at it Celebrity Death Match style.
Well I've never played a Smash Bro's game so can't really comment but I would assume people play it because they enjoy the gameplay (which is quite different for just about everything else) rather than looking forward to punching Princess Peach.