Kirby's positional advantage on RC's boat is below the platforms, close to the wall. We can string moves with platforms nearby, and the wall stops us from sliding too far if someone hits our shield with a strong attack, not to mention the obvious "I'm below you" stuff that Kirby can do if an opponent is on higher ground. That's Kirby's optimal position against Ike, since being close to the sides of the ship means we'll be close to the blastzones, and good ol' Ike likes to hit hard.
Kirby's positional advantage on the side-scrolling area on top of RC is simply remaining ahead of Ike. Eventually we'll all have to go down, and Ike will be forced to go down on the falling platforms at the left side of the big block, or he will go down on the jump-through platforms on its right side. Either choice is a good thing for Kirby, since you're indirectly approaching us, else you risk dying off the top...
That's our optimal positions, but we can do very well throughout the whole stage, provided we stay ahead and force you to indirectly approach us.