Katakuri was a breath of fresh air for a few reasons.
Every one in WCI was incompetent, and even when they seemed like they weren't, their sole purpose was to hype their captain Big Mom (Cracker is an example of this, his scenes served nothing more than to hype up his captain and her territory). Katakuri was the sole character on WCI who wasn't just an extension or a hype tool for his captain. Since his introduction in WCI, all of his actions (and others who hyped him) were built for him and him alone. To hype himself and build himself,
not Big Mom (in fact his hype was given a priority over her). Not just for strength but as a character himself, he felt
real, like an actual individual entity in a massive crew where others were just nothing more than fragments of Big Mom and WCI scattered around in character form. Katakuri was gold among the dross, competent and distinct among his peers.
So Oda kept building him up, calm, collected, cool, the perfect character. His character archetype already solidified, his stock already on the rise....and then bam! We see this perfect character, sitting there looking like a goofball with his mouth wide open feeding on donuts with his back rested, akin to you catching your role model in their room masturbating. Butt naked and completely vulnerable. One of the most shocking moments in One Piece history, and the immediate aftermath was bone chilling. The calmest and most understanding character, seething with rage, brutally murders his men without hesitation. A complete 180, only Oda would have the balls to pull off something like that on an "edge-lord" archetype. Did he
need this? Not really, simplicity can work wonders as well, he was already fine the way he was but just like that, this moment added layers of depth to his character.
Even before that though, in battle Katakuri was unique. Katakuri from the jump was treating his fight against Luffy as a 1v1 show of dominance, his fight started with him using Luffy's own arsenal against him but bigger, faster, and stronger. Absolutely embarrassed him off the jump. Aside from that, his unwillingness to let Luffy enter Gear 4 broke the meta. Finally an opponent wasn't all empty talk when it came to "not underestimating" Luffy. People love to bring up him doing exactly that (underestimating Luffy) by leaving him, but this was handled as well as a PIS moment could be imo. Burying Luffy was the convenient thing to do and the reason for doing so was because he had already tried to deal a lethal blow to Luffy with Mogura, but that wasn't working, so overwhelminging Luffy with his awakening was the fastest way of getting rid of him (atleast in Kat's head). Like any temptation or craving, Kat’s mind being preoccupied by his daily routine already running late, kept him from the patience he needed to see through to Luffy’s death (in the first place Katakuri had no idea how big of a connoisseur Luffy was, burying Luffy in food is only benefiting Luffy, most people would suffocate).
In stark contrast to this Doflamingo seemed like a contradictory mess. He started off the arc noting Crocodile's defeat, making sure he learns from the danger of underestimating his opponent, yet when the time to back that up came, he was letting Luffy power up in his face and laughing at it, and then even after knowing the strength of G4 and the risks of taking it on, he directly challenges it with a "my dick is bigger than yours" moment gearing up for a final clash, instead of using his head. Leaving his statements holding no weight, and falling back into the typical One Piece villain formula. Heck even before this, he had Law completely in his hands with sea stone cuffs, Luffy right there in front of him, and Pica at his disposal. What does he do? Lets them all free and starts a treasure hunt game. Only for Pica to get defeated, Law to come back in better shape with his fellow WG member. Yea so much for not underestimating his enemies. Doflamingo was pretty much a pseudo intellectual masked as a cunning meticulous planner, he came off as a bit of a fraud because of this.
Katakuri on the other hand, from early on in the fight had been laid down as a "fraud", the scene of him eating donuts completely laid the groundwork for a potential change in approach, because now we know he isn't exactly what we were made to believe. His entire fabric had been undone infront of our eyes, so anything from that point on was a question mark. Anything went. And no, him stabbing himself wasn't the real change, from the get go Katakuri was big on this being a 1v1 with him and Luffy [
1]. So it was obvious that interference on that caliber was not going to sit well with him. The
real difference in approach from the beginning and the end was how he let Luffy get back up and power up, vs earlier on where he refused to let him power up. But again like I said, after the merienda, anything goes. Half the reason he was ending the fight ASAP earlier was to have his meal, Luffy powering up does not help his case in his head. With all of that out of the way, he wasn't worried about ending things as soon as possible, his priorities and focus shifted. This falls in line with Big Mom and how her kingdom had been run in the first place. Big Mom does not have this many strong individuals under her because they respect her as a Captain, Big Mom is their biological mother, these kids were given no choice to ever be a part of her crew to begin with. The day they were born they were already pushed into this storm. And this really showed throughout the arc, with a lot of the crews loyalty coming into question. Her way of ruling was her own downfall.
All of these reasons are why I believe Katakuri's battle was more believable (no I'm not talking about Luffy's endurance, but rather Kat's actions) than the contradictory decision making Doflamingo kept displaying during Dressrosa. Both Katakuri and Doflamingo have badass character designs and swagger, but at the end of the day as much as a special case Doflamingo prepped himself up to be, he ended up acting like your generic One Piece villain, no different than his predecessors (Croc, Enel etc). The execution behind that is the reason
I enjoyed Katakuri more as a character, he was truly a breath of fresh air from the norm.