I would say being an important villain contributes a lot to K. Rool, but then again he's commonly replaced by other villains.
Well, not necessarily. It depends on what you mean by
commonly.
He was the final boss (and main antagonist) in DKC1, DKC2, DKC3, and DK64. Those four games are part of the main DK series. You also have the many spin-offs***; K Rool is playable in two (Barrel Blast and Mario Sluggers), and appears as the main antagonist in King of Swing and Jungle Climber.
The current Retro series (Returns) marks a sort of revival of the prime DK franchises, after years of spin-offs (Konga, Barrel Blast, Jungle Climber, King of Swing, etc.). K Rool has not appeared in the two Retro games yet; we had Tiki Tong in DKCR, and Lord Fredrik in Tropical Freeze. So in reality, K Rool has only been replaced twice.
In unrelated trivia, in the K Rool thread, many have forward the notion that Fredrik (i.e. the Ice Walrus Viking) is essentially a K Rool expy, given that Fredrik's moves in his boss fight are highly reminiscent of K Rool's behaviour in past DKC titles. Also, Retro made a sly reference to K Rool in Tropical Freeze (in one of the level backgrounds). While intriguing, neither of these things don't do anything bolster K Rool's Smash prospects.
So, we have six central DK titles, in which K Rool appears 4/6 (or 66% of the time), while Dixie appears 3/6 (DKC2, DKC3, Tropical Freeze). For spin-offs, K Rool appears in 4/10 of them, and Dixie 9/10 (she is absent from Donkey Konga 1). I imagine that the central DK titles have more innate significance than the spin-offs, but Dixie has far more presence in the spin-offs, which balances things out (and possibly tips them in her favour to a degree).
Anyway, take what you will from all this. I hope you found this informative; I checked the DK wiki just to double-check my information.
***There is Konga 1, 2, and 3, Jungle Climber, King of Swing, Barrel Blast, Diddy Kong Racing DS, Super Mario Baseball and Sluggers, and Mario Hoops 3-on-3. K Rool and Dixie appear either as playable characters or make cameos (in which K Rool appears as the antagonist for two of the games).