Rubber and Mochi (or any food) are two completely weird things to compare. Even as Devil Fruits, Luffy is a quintessential paramecia, while Katakuri is only a paramecia in name, since foods can't be logias. Only in the context of Japanese culture would those two draw parallels between them. So what makes them similar? Obviously, it's that both rubber and mochi can be stretched. In fact, Katakuri can stretch even more than Luffy. However, Luffy took pride in his Devil Fruit, and believed Rubber to be the Superior Devil Fruit: So what makes them different? According to Luffy, it's that "rubber always bounces back". In true cliche, shonen fashion, he's a character that always gets back up after hardship. This doesn't mean that Katakuri's arm doesn't return to normal after being stretched. It's that rubber's tendency to do so, is immensely higher. Obviously, this was a nod to their personality and strengths as well. Luffy doesn't mind hitting the ground. He thrives on it. It's only getting back up that matters. Like his powers, Katakuri is afraid if he hits the ground, he won't be able to return back up again. Katakuri is afraid of failure. Conclusion: When we think of rubber, we think being able to stretch. Even with Luffy's Devil Fruit, what is the first thing that comes to mind? He can stretch his arms. But here was Oda's way of saying, that stretching is not what makes the Devil Fruit special, because Katakuri's DF can do an even better job at stretching. It's the recoil/rebound effect of rubber, that makes it strong. And the more it's stretched, the more powerful it is. The bigger the failures (Ace), the stronger the successes. Luffy represented an alternate version of Katakuri, if Katakuri took risks in his life. Luffy grows from failure, Katakuri is afraid of it. (After all, this is a man with King's Haki, meaning he has huge ambitions) So, instead of complaining that Luffy doesn't stay down, you have to realize that that was the whole point of the fight. It wasn't Oda nerfing Katakuri or giving Luffy plot armour. It's Oda showcasing the true power of the Gomu Gomu no Mi. No matter how many beatings it takes, it always rises back up. The only way to subdue it is by overwhelming lethal force, not 1001 punches. What Luffy taught Katakuri is that it's ok to fail. True "strength" is not ever failing. It's getting back up from failure.